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DIY LPG Installation Print E-mail

Do-It-Yourself Automotive LPG Conversion

by Tom Jennings

Reprinted from: http://wps.com/LPG/LPG-book-final.html#SYSTEM%20DESIGN

World Power Systems, San Francisco CA

Fourth edition, Nov 98. Third edition, Apr 97. Second edition March 1994. Original printing April 1991. Copyright Tom Jennings 1991, 1994, 1998. All rights reserved.

Do-It-Yourself Automotive LPG Conversion

Table of Contents


The vehicle featured in this LPG fuel conversion project -- my 1963 Rambler Classic 550 Station Wagon, with a late-model (well, 1970) 232 ci. 6 cylinder engine. 3,150 lbs. of finely wrought steel. Substitute your favorite car here.

There already is a practical alternative to gasoline as a motor vehicle fuel. It's cheap, doesn't require exotic equipment, and is retro-fittable to many existing vehicles. It's street-legal, even in California. It produces half the carbon dioxide of gasoline, vastly reduces other pollutants including dirty motor oil. It's Liquefied Petroleum Gas, or LPG for short.

This book describes how I built my LPG-powered automobile, provides some background information, and lists my experiences in operating an alternative-fuel car day to day for over five years. My goal is to outline the process I went through, provide the basic technical information to build your own LPG-powered vehicle, and give you an idea of what it's like to use it in real-life terms.


Liquefied Petroleum Gas, or LPG, is the correct name for what's commonly called propane, butane or bottle gas. It's actually a mixture of propane, butane, ethane, methane, and other gases. Physically, it's a gas at room temperature, and compresses into a liquid at reasonable temperatures and pressures, which is why it's called Liquefied Petroleum Gas. It's odorless, but has an odorizer added so that you can smell leaks.

It's non-toxic enough to cook with indoors and heat your house; indoor warehouse forklifts use it without ventilation; Supermarkets buff their floors with a propane-powered floor buffer. It's also a claimed environmentally-safe aerosol propellant, quickly replacing freon in many cases.

There are a number of reasons why I wanted an LPG car: low tailpipe pollution; extremely long engine life; 250,000 miles is routine, 400,000 miles rumored; vastly reduced oil consumption, and waste oil is less toxic than a gasoline-engined car; and it allows turning larger, simpler, cheaper, less-efficient vehicles into practical alternative fuel vehicles. It's also technically interesting, and I simply like traveling unusual paths.

After nearly ten years of driving relying on my LPG car as primary transportation, I am hard-pressed to find LPG's drawbacks. Of the limitations of my car compared to an equivalent gasoline car, all are because mine is homemade.


For a sense of scale, I'll too-briefly compare some well-known energy sources to a reasonably efficient gasoline automobile. A 1994 Ford Festiva will carry 4 people plus itself (total: 2200 lbs or so) about 50 miles on one gallon of gasoline under best conditions. One gallon of gasoline contains about 37 kilowatt/hours (KWH), so for comparison purposes this is what I'll use.

(A kilowatt is 1,000 watts; expending 1,000 watts (heating water, running a motor, etc) continuously for one hour is a measure of work done, and a kilowatt/hour (KWH) is a decent human-scaled amount of work; one ``horsepower'' is 746 watts.)

ONE PERSON pedaling an efficient bicycle fairly hard can generate about 100 watts of energy; kept up for an hour is a fair amount of work, and in techie terms, is called, appropriately enough, 100 Watt/Hours, or 1/10th of a kilowatt/hour. Do this for 370 hours, and you're there.

ELECTRICITY in San Francisco costs about 12 cents per kilowatt/hour. (Electrical utility power is highly subsidized; actual system-wide cost is much more.) That gallon of gas would be about $4.44.

Storing electricity is difficult and expensive. To store that 37 KWH-worth of energy, you would need about $4800 worth of hi-tech lead chloride cells, weighing 2400 lbs -- you'd still have to generate the energy to put in them. Electric Vehicles (Evs) are great, but someone else's story.

SOLAR POWER form solar panels (PhotoVoltaic, or PV) generate electricity ``for free'', once you own them. Today's state-of-the-art PV technology converts about 13% of the sun's energy striking them to electricity when aimed properly; a panel capable of generating 37KWH in 8-hours worth of bright sunlight would cost $33,500 (96 ARCO M-75's, assuming a better price than the Real Goods 1990 Sourcebook).

Good-size solar PV panel, looks like 180 Watts or so. Installed in a well-designed system in the right part of the world, adequate to power a very small house. (See Home Power and Real Goods in SUPPLIERS & SOURCES.)

HYDROGEN sounds like the perfect fuel until you try to make it or store it. In theory a non-polluting source, in practice it has to come from somewhere, and guess what -- commercially available hydrogen is usually made from petroleum-derived methane. In the laboratory it can be made by breaking water molecules into its component hydrogen and oxygen, but it that takes huge amounts of energy, far more than you recover when you burn the oxygen and hydrogen later for power. Even with a perfect storage medium and 100% efficient fuel cell, you might as well simply store the energy you'd use for wrestling with hydrogen atoms and use it directly to do the work you originally intended. One pound of hydrogen contains 36KWH. It's volume depends on the storage medium.

METHANOL (and ETHANOL) is pretty powerful stuff; at 23KWH a gallon similar enough to gasoline; 16 gallons of methanol has the same energy as 10 gallons of gasoline. It can also be produced from fermentation of organic matter, and apparently in Brazil lots of commercial fuel alcohol is, making it a renewable energy source there. Here in the U.S. most methanol production is from petroleum and lumber-industry byproducts. It's attractive as a motor fuel because it fits in existing fuel-dispensing systems and equipment, but not a very clean fuel, pollution-wise; see below.

PROPANE -- LPG is yet another ``byproduct'' of the petroleum distillation process. By a lovely coincidence of physics these gases liquefy at reasonable temperatures and pressures, so that one gallon of LPG (liquified; about 270 gallons in gaseous form) contains 27KWH of energy; 13 gallons of LPG is equivalent to 10 gallons of gasoline, even though to use as a fuel they are handled very differently.

lbs/gal  4.24    6.4      6.84 
BTU/gal  91,500  126,000  79,400
Boils at -44F    85-390F  149


From an ``alternative'' point of view, alcohol (methanol or ethanol) seems to be the winner at first glance, because it is a potentially renewable fuel; if made from fermented organics such as plants, no more carbon dioxide is produced in combustion than is bound when the plants are grown or replenished.

Unfortunately there are a number of problems with using alcohol for existing applications. One problem is the simple fact that no organically-produced methanol is available in the US, and home production of decent quantities is not a trivial task. Also, methanol, because it is a liquid, simply does not burn completely in an internal combustion engine. The theoretical products of methanol combustion, as for gasoline combustion, are carbon dioxide and water. In real life there is unburnt fuel, carbon monoxide, nitrogen compounds, and various hydrocarbon fragments from incomplete combustion combined with the high pressures and temperatures of the cylinder combustion process.

Basically the problem is that liquid fuels are liquid -- even after vaporization in the carburetor, swirling in the cylinder head etc, fuel droplets remain unburnt, because of the short time given for each cylinder to fire to operate the engine.

Gaseous fuels simply don't have this problem -- the individual fuel molecules are by nature separate, and burn very nearly completely. This is the major reason that LPG is such a good motor fuel.

Though the sheer volume of gas needed is intimidating, fermentation methane, dried and compressed into liquid, would be at once renewable and non-polluting. Most likely you could have the same vehicle operate on commercial LPG (long trips), or methane from fermentation.

Also in LPG's favor is that pound for pound, it contains less carbon than gasoline or methanol (more of it's energy is bound up in hydrogen atoms than carbon atoms) -- an LPG powered vehicle produces approximately half the CO (carbon monoxide) and CO2 per mile, though only slightly less nitrogen compounds (created when the high volume of atmospheric nitrogen, some 70% of the air, is subjected to engine cylinder temperatures and pressures).


Having now used both gasoline and LPG, I far prefer handling LPG. My now-infrequent trips to gas stations (in other peoples' vehicles) are like visits to toxic waste sites. It's amazing what we get used to!

A gasoline spill, by accident, or more ordinary dripping hoses at a gas station, is a toxic nightmare. In addition to being exceedingly flammable, gasoline runs into streams and soak into the soil, poisoning everything it touches. Its vapors are explosive. It's a known carcinogen. Even if the fire hazard is overcome, the toxicity lingers on.

An LPG spill is just as flammable (or it wouldn't be a useful fuel!), but instead of poisoning the environment, it boils away as fast as it can absorb heat. LPG is non-toxic. It's vapors can collect in low spots, where it is a fire hazard, since it is heavier than air. Once the gas has dissipated, there is no further hazard. Spilled liquid, while rare enough in anything but extreme circumstances, will give you nasty frostbite.

Basically, we're just used to gasoline, most of us growing up assuming the stuff is everywhere, and we have systems to deal with it. LPG is ``new'' and somewhat alien, but on the whole no worse, and once you're used to it, safe and easy to handle.


One way to think about the physical properties of LPG is to think about how you'd have to handle water and steam if the temperature around us was 250 degrees (F). If you were to pour water on the ground at that temperature, it would immediately boil into vapor and ``disappear'' into the atmosphere. To store it, you would need to keep it under pressure, in cylinders strong enough to hold it, plus a safety margin. This is exactly how LPG acts in our normal range of temperatures. The boiling point of any liquid (the temperature at which it vaporizes) increases with pressure; to keep water in it's liquid state at 300 degree F you'd have to keep it in a container at just over 67 pounds/square inch (PSI); at 350 degrees F, 135 PSI.

LPG acts the same, but at much lower temperatures; at 80 degrees F it must be kept at 130 PSI to keep it liquid. The fact that LPG fuel is a gas, and not a liquid, at first seems like a liability, but is it's strong point, as you'll see later.


I completed the LPG conversion of my car in October, 1990. When I first wrote this book, in April 1990, I had only limited experience with the car, which was covered admittedly poorly in the first edition. Since that time, I've taken a number of long road trips. In brief: everything went better than expected. The car ran great at all altitudes (sea level to 9900 feet), in hot (110 degree) and cold (35 degree) weather. Fuel availability was good in most areas during daylight hours, poor at night, and in a few places hard to get. At no time did I run out of fuel, though I admit I worried about it a lot, not always needlessly.

This seems an appropriate place to mention a lot of information that doesn't fit elsewhere. I'll simply describe my subjective experiences of driving an LPG car on my various road trips.

The first thing everyone asks (after ``doesn't it blow up?'') is ``where do you buy fuel?''. We're so used to seeing gasoline stations everywhere, we overlook the shiny white tanks that LPG generally comes in.

Fueling up with LPG is a completely different experience from buying gasoline. With gas stations at nearly every Interstate exit and highway intersection, the thought of having to search for LPG is daunting. In fact, it's not that bad. LPG is very common in rural areas; you start noticing the telltale white tanks all over. Many rural gas stations have smallish LPG tanks off to one side.

If your driving habits include constantly running on a near-empty tank, and getting $2 worth of fuel in the middle of the night at the last possible, digging change out from under the seats, you'll have to either change your habits or stick to gasoline.

On the Interstates, LPG stations are found in truck stops, or on the outskirts of town. Nearly every large town or city has one of the regional or national distributors, who generally have the lowest prices.

Finding LPG during daylight hours is easy, early eves not so easy, late at night difficult in rural areas, since most LPG there is non-vehicular. However on a recent trip to Denver, I was able to easily drive all night, fueling up at truck stops and small towns in Nevada.

LPG is sold by the gallon; prices vary widely. The lowest prices are at the distributors themselves, and the highest at the local mom'n'pop store with a 100 gallon tank for the RV'ers. At a time when gasoline was about $1.15/gallon, LPG at a distributor in New Mexico was $0.95/gallon, and a mom'n'pop in California $1.35/gallon. The lowest I've ever paid was $0.59, the highest $1.50; as you can see, the prices vary widely. During this time, I was typically paying $1.20--$1.30/gallon at a typical gas station's LPG service.

Fuel availability seems to have noticably improved since 1990; maybe I'm just gettingbetter at spotting them out of the corner of my eye, but it really does appear to be proliferating, and I'm uncertain why; there's certainly not many more LPG cars out on the road that I can see.


I've taken a half-dozen or so long (1,000 mile or longer) road trips since the first edition of this book. In over 14,000 miles of driving, I had one mechanical failure due to my LPG system. (I'll not mention the rock that cracked my windshield on an Arizona highway in February, the headlight switch that dropped dead just as I pulled onto a Nevada highway at night, or the taillight fuse that blew in California when my clock decided to fail.)

The failure was actually a problem built-in from the start, that caused me grief on a number of trips, and incorrectly diagnosed in the first edition (how embarrassing). See the MY PROBLEMS section for details; but briefly, I had two check valves in the fuel-filler system that wanted only one, and they fought each other. Once I installed the correct part the problem went away forever. The parts supplier was partly at fault, as I bought the complete system from them. I had other problems with this supplier as well.

Running out of fuel on the road would be a major inconvenience, as even AAA doesn't seem to know anything about LPG motor fuel. I've worked out a simple, safe and reliable ``gas-can'' system; see below for details. I've intentionally used it a dozen times (not once have I ever been forced to go fetch fuel) and it works just great.

As of January 1990, California LPG vehicles need to be ``smogged''. The car simply has to meet the smog specifications for that model and year, which is amazingly easy. The testing machine has a setting for ``propane'' fuel. When the auto center I went to found out the fuel was LPG, they panicked. It took a call to the Smog Board arbitrator to get it all straightened out. California is the most stringent state in the U.S. for smog controls.

Most LPG station attendents I ask say they fill no, or very few, LPG vehicles, and most of those are pickups; one on Route 5 said one or two a month.

In California you can also get a sales tax exemption -- a sticker you apply to your window, that exempts you from paying sales tax on LPG motor fuel. It costs $36 (Jan 1991), is good for one calendar year. Call the Franchise Tax Board for details. I had to explain to them it was for a passenger car; the person on the phone started rattling off commercial GVW requirements. Presumably most of their customers are truck fleet owners. I've found that few LPG dealers even know they're supposed to be charging road tax, so it's become moot. I never bothered with a tax-exampt sticker.

On the road, I asked a veteran Canadian RV'er with an LPG pickup (150 gal. tank!), what's the situation on LPG availability around the U.S. He said California was the worst. (If this is the worst, things aren't so bad.) I wonder about the dense NorthEast.

A quick scan of the Yellow Pages under ``GAS - PROPANE'' for San Francisco lists 15 locations.

U-HAUL's national ``RV World'' stores always carry LPG, and some are open 7 days. Most campsites carry LPG too.

All KOA Kampgrounds have LPG, some 24 hrs/day.

On a trip down the Northern California's Route 101, I found LPG to be plentiful, though somewhat spotty at night.

Join AAA and request their list of campgrounds for areas you travel in. The campsite listings list propane services. Bug AAA about LPG fuel-availability guides, vehicles, service; they need to be told we're out here.

Other avenues to follow that I haven't yet -- truck stop franchise chains, oil companies, service companies, all want to sell you their goods and services. A call or letter asking where to buy their products should be fruitful.

Engine oil stays yellow and odor free 10,000 miles after an oil change. It never gets as dark and smelly as it did on gasoline. Many LPG dealers say that 16,000 miles is the recommended oil change interval; I just can't bring myself to go that far, and oil is cheaper than engines, I generally change mine at 10,000 miles or six months. There are LPG-specific motor oils, and the back of the can propaganda talks about reduced acids from longer oil change intervals, and reduced shear damage; while this hardly constitutes oil chemistry research I'll take the hint and stick to 10,000 miles per oil change!

A typical LPG fuel filler with the locking door open. The round Acme fuel connector is in the center, and the gas vent in the lower left corner.


This construction book is only the first phase of my project, and unfortunately not the interesting part, regarding truly alternative fuels. I don't have bags of money, and I have to arrange things in a manner I can actually get them done, and still accomplish my long-term goals.

Access to decent test equipment is the obstacle to getting real numbers on emissions. Commercial smog stations have $30,000+ invested in their machine, and will charge you big bucks ($45 per test) in order to recoup the expense. Home type equipment I don't have, and haven't yet investigated, as to accuracy and sensitivity -- it must be able to measure extremely low levels of hydrocarbons (HCs) and carbon monoxide (CO).

From a non-global-system perspective, LPG is a winner. Your vehicle will last far, far longer, maintenance is simpler and less frequent, so you can at least slow down your part of the consume/discard cycle. And you can do it today.

From a global-system point of view it's definitely a short-term gain, as you immediately eliminate the toxic parts of vehicle emissions. At the moment it is not a major motor fuel, and you get to work in the margins of the petroleum industry, and if you can produce your own fuel gases, get out of the loop entirely.


The ``typical'' older American car or light truck is a perfect candidate for LPG, as they tend to have large displacement, carbureted, water-cooled engines. Four, six, eight cylinder doesn't matter.

Air-cooled engines would require a different converter/regulator design. I don't know what an LPG carburetion system for a fuel-injected engine would look like. Better ask an LPG service center.

I'll assume that you more or less understand the major systems in an ordinary automobile -- an internal combustion engine that converts stored chemical energy (fuel) into motion by burning it, a transmission and such to move the wheels, suspension, brakes, and all that to make it practical. The only thing different in an LPG vehicle is the fuel system.

A basic LPG fuel system isn't really much different than a gasoline system. There's a fuel tank, some hoses, a fuel filter, and a carburetor. The biggest difference is that LPG is under pressure, so the tank is heavy gauge steel with various safety features, the filter doubles as a fuel shutoff, and there's a regulator/converter that turns the liquid LPG to a gas explicitly before entering the carburetor.

The hose used to carry LPG to the engine compartment is heavy neoprene covered with a stainless steel mesh, with a fabric covering. It's rather bulky (about 3/4 inch), but rugged and designed for 1750 pounds per square inch (PSI). An LPG tank left in the sun will generate a few hundred PSI, so there's more than enough safety margin there.


You have the choice of installing an LPG only or dual LPG/gasoline system. As you might guess, the dual-fuel setup is a compromise; you'll get maximum performance, simplicity and economy with the single fuel setup. That is what I chose. I talked to the owner of a 1970 GMC pickup who converted to dual fuel when the truck was new (181,000 miles, no motor overhaul yet) and says he runs mainly LPG, and gasoline when LPG isn't convenient. You may have trouble with the carburetor gaskets drying out and such; talk to the LPG equipment dealer for details. The GMC owner had no complaints, so maybe all that's overrated.


LPG experts will tell you two things you will need to make it all work -- a good ignition system and a good cooling system. And they're absolutely right. LPG requires a good hot spark, and an ignition marginally OK for gasoline will run terribly on LPG. I had a classic textbook example of this happen to me; read about it in the PROBLEMS section.

An electronic ignition is probably a must. My car had old-fashioned points and spark coil, to which I added an Autotronics Multiple Spark Discharge ignition, which cost me $89 in 1986. It works flawlessly, simple to install, and a set of points now lasts 50,000 miles.

You need a good cooling system, not because the engine runs any hotter, but because the LPG regulator/converter uses engine coolant to provide the heat to convert the liquid LPG to a gas. (Even ``cold'' water works -- it just has to keep it above freezing.)

An LPG conversion does not require any special modifications to the engine. Since LPG has fewer BTU's per pound than gasoline, and the engine is designed to take in a fixed volume of fuel/air mixture, you'll get about 10% less horsepower for a given engine, at full-throttle. To help compensate, LPG has an octane equivalent of about 110 -- meaning you can advance the ignition timing a lot. The actual loss depends on the specific engine, but it will probably not be noticeable.



I built my engine specifically to run LPG, back in 1988. Since I knew it would be a while before I could afford all the LPG equipment and get around to installing it, the motor had to run on gasoline also. The compromise was easy, and not much of a compromise.

There are three areas affected by fuel choice -- compression ratio, cam timing, and exhaust valves and seats. The biggest compromise was compression. 12 to 1 isn't too high for propane, but is too high for gasoline. My engine was about 8 to 1 stock. The machinist (Mike at Folsom Auto Supply) found a stock piston from another engine that raised compression to about 10 to 1, a decent compromise for no additional cost; since I was boring the cylinders .030" oversize I had to buy new pistons anyways.

I had a camshaft custom made by Crower Cams, though any good cam grinder should be able to do it. I told them over the phone my application (to their credit they didn't laugh) and told 'em dead stock except for the compression ratio, and, importantly, to move the torque peak much lower (I never rev the engine beyond 3500 RPM.) They came up with the following: 240 degree duration intake and exhaust, .390" valve lift, with intake valves open at -13 ATDC, close 15 ABDC, exhaust valves open at 23 BBDC, close at -21 BTDC.

[1994 note: This particular camshaft was a terrible choice; the cam grinder designed a low-vacuum camshaft that's proven disastrous with LPG carburetors, which use engine vacuum at low speeds to determine fuel/air ratios, and the car runs constantly rich with poor mileage. The camshaft was replaced with factory-stock in late 94. The rest of the original camshaft discussion has been deleted.]

The last item to improve is to use hard exhaust valves and valve seats, such as Stellite. Hard valves were not available for my engine, alas, though I was able to get hard valve seats. The ignition temperature of LPG is somewhat higher, though not enough to worry about if you can't get them; I doubt it will make even a 5% difference in valve life.

[1998 note: the engine now needs it's valves rebuilt or replaced, with about 90,000 miles on them. It runs fine, but mileage is dropping. I have annual cylinder compression data starting 1991, when it was 140-145 lbs/sqin; it's now dropped to 115-125 lbs/sqin, which increased slightly with a squirt of oil. I may renew my search for hard valves for this engine.]


There seems to be two major manufacturers of LPG systems, Impco and Century. Impco seems to be the simplest and easiest to install, and is what I used.

An LPG system consists of a filter/shutoff, converter/regulator, and carburetor chosen to fit engine size, and hoses, connectors and other more common components. Component costs don't vary much with size; if I remember correctly the next-size-up regulator/converter was $10 more.

LPG fuel tank as installed in my Rambler. This tank was meant for a large van, and is rated at 20 gallon capacity; in fact, it only holds 18 gallons of fuel. (The photo greatly exaggerates how much the tank actually hangs down.)


The fuel tank is the single most expensive component, and the hardest to fit. Since my car is LPG-only, I removed the gasoline tank and put the LPG tank there. If you are doing the installation yourself, you'll have to carefully choose a location. Most passenger car and van installations will use ``siamese tanks''; two smaller tanks welded side by side to make a larger, more compact tank. Alas, they are also more expensive. I was able to get one for cheap because the dealer had a stack of them from a contract that fell through. Each installation will be different. Visit the dealer with tape measure in hand.

[1994 note: I've noticed that wrecking yards devoted to trucks and commercial vehicles generally have lots of used LPG tanks. Since there's very little market for them you can get 'em dirt cheap. A tank nearly identical to my new $400 tank was available for $50 used. Live and learn. Make sure it's not rusted!]

[1998 note: Earlier this year I replaced the 18 gallon tank with a brand new 35 gallon tank (28 gallon fuel capacity). What a refreshing change! At about 14 MPG propane average-worst-case mileage, it gives me a 400 mile range full-to-empty. To do this I had to sacrifice my spare tire compartment, and will locate a "continental kit" type bracket to mount it externally. Since this car is mainly for road trips it's not a problem for me. My next conversion will be to my "new" 1975 Gremlin X V8, where I should be able to fit the same 35-gallon tank and keep the spare inside, too.]

With one exception, the LPG fuel tank must be installed outside or under the vehicle, and all hoses outside the passenger compartment. Safety is the issue, and no different than a gasoline-powered vehicle. The one exception is trunk mounting of the LPG fuel tank, especially in sedan-type automobiles with the gasoline tank still in place. In this case, the trunk area {\it must} be sealed off from the passenger compartment with foam, and vents to the outside cut in the body at the lowest point, to allow potential LPG vapors to escape. (There are also tanks and add-on devices to completely enclose the hoses and fittings in a ventable flexible container, such that the tank could be in the trunk safely.)

A good start for a new tank is the Manchester Tank Co. in Lynwood, CA. They publish a catalog of their tanks. My new 35 gallon tank cost $595.00 (June 98).

The filter/shutoff does what it's name implies. The filter element lasts 100,000 miles, and mainly keeps out junk that may have been in the tank when it was made; the fuel itself is quite clean. Since the fuel is under pressure, the shutoff turns off the fuel when the engine is not operating. The Impco system uses engine vacuum to do this; some models, and the Century brand, use a more complex electrical system.

The fuel hose is high-pressure stainless-steel-mesh reinforced neoprene, and the dealer should stock just about any size required. It's fairly expensive, but make sure you get enough -- the only thing worse than too long is too short! Under no circumstances should you run fuel lines -- LPG or gasoline -- through the passenger compartment.

You will also need a handful of connectors for the hose. They are easy to install -- after cutting the hose-end nice and square and clean, you screw on the outer part counter clockwise first, then screw the inner part into that, tighten snugly ( Before and after photo here (9K).).

The carburetor is fairly easy to install. You'll get an adapter to fit the intake manifold and the throttle lever is made to accommodate a custom fit. It took me a few hours to lay it all out, and make a throttle linkage with hand tools.

Last but not least -- the fuel gauge. You can use your existing in-dash fuel gauge. There is a wide range of tank sender units to fit most cars, that connect to the original wiring and work in the usual manner. A 90-Ohm unit fits most cars. You'll also need a fistful of sheet metal screws and plumbers tape, for mounting the fuel hose to the car. A decently stocked ``junkbox'' helps, though most everything else you'll need will come with the components you buy.

Overall view of the engine compartment (38K). The convertor/regulator (a) and filter/shutoff (b) are visible on the right inner fender, and the carburetor (c), in the foreground, has it's air filter in place.


Figure out where the LPG components will go. Placement isn't critical, but you will want to keep these things in mind.

Closeup of my LPG fuel tank installation. A very tight fit.

1. Mounting the tank will be the hardest part; gasoline tanks are made-to-fit the car, and LP tanks don't come in such convenient shapes. Mine fit fairly well, but hang down a bit).

2. Running the LPG hose requires a bit of care. You don't want it exposed where running over a rock or curb could crush it, and you should attach it to the underside of the car every foot or so, as any flexing will eventually wear through the protective covering. After a week of operation, I found mine had been chafing at a few places. Attaching it every 12 inches with plumbers tape solved that.

3. Mount the regulator as low as possible, (less sensitive to coolant level) and close to the carburetor. My installation came out pretty neatly, with a one foot section of hose between the filter and regulator, and an 18 inch piece of vapor hose to the carburetor. (For most installations you can simply connect the filter/shutoff and regulator/converter together as a unit with a short pipe nipple instead of hose.)

Close up photo (31K) of the major components. Regulator/converter (a) and filter/shutoff (b) mounted on the inner fender. Note the water connections coming from front of the engine (left) and heading up to the heater on the firewall. The LPG hose is at (c) and the vapor hose to the carburetor is (d).

4. The water connections aren't fussy; as long as you get decent water flow, and keep the radiator full, you're all set. (I plumbed min in parallel with my heater, and installed a water valve to restrict the hot water through the regulator/converter to a trickle; this leaves plenty of hot water for the heater.)

[1998 note: my original water system was stupid. It's simply better to by the "H" fitting, basically two parallel pipes with a small cross pipe; you put the two larger legs in series with the hot water to the heater; the small bypass assures that a small amount of hot water flows even when the heater is off. The converter is simply connected in series with the heater.

5. Carburetor installation is simple. You may have to fabricate a throttle linkage, but it's designed to be easy, and you can get adapters for nearly anything. The Impco carburetor has a throttle lever that fits on either side of the throttle shaft, and there are many adapters available that rotate the carburetor 90 degrees. It's also far shorter and more compact than a gasoline carburetor, so there will probably be no interference with other components.

Carburetor, minus air filter, with the vapor hose coming over the top of the engine. Note the regulator/converter and filter/shutoff barely visible in the background.

6. The fuel filler system is more complex than for gasoline, but still simple. It consists of a brass threaded ``Acme'' adapter, and a small gas vent valve. Get the more expensive model with a locking cover; the other style available is for fork trucks and such, and any idiot wandering by can let all your fuel out by opening the gas vent valve! The cover type also looks much better.

Fuel-filler system with the locked door open. The Acme filler connector is in the center; the vent valve is in the lower left corner.


When you first install the system, it will be empty of fuel. I used my ``gas-can'' (see below) to purge the tank and put a gallon or two of fuel in the system and check for leaks.

First, use plumbers' bubble-leak detector or soap solution to check for leaks. I used a pump sprayer with Dr. Bronners liquid soap and water.

1. Open the LPG liquid valve, and look, listen and smell for leaks. A squirt of soap at all connectors helps. If you find a leak, try tightening a bit; if a little is good a lot is not better; you'll just wreck things. Check for leaks up to the filter/shutoff.

2. Start the car -- no fancy preliminaries. The regulator/converter will purge itself of air, and it should start within 5 - 10 seconds. The throttle must be opened a small amount.

You will have to tweak the idle speed and mixture. I set the idle too high at first, and adjusted it after it was running.

3. At this point, check for leaks after the filter/shutoff.

4. Tune up the car normally. No special settings are required, and for emissions/legal purposes, none desired. Set the ignition timing dead stock. If the car was previously in tune, all you should need to do is set the idle mixture and speed on the new carburetor.

5. After a thousand miles or so, check the spark plugs. Spark plugs will last considerably longer, as there are no fuel additives to build up on the insulator during combustion. Check for the right heat-range plug -- if the insulator color is too white, or if it looks burnt, get plugs one range colder.


There are some problems unique to LPG fuel systems, and because the components are mechanical devices, they can fail. Here's some tips on troubleshooting and repair.

In general, if you find frost on LPG components (liquid service valve, filter/shutoff, regulator/converter) you have a problem. If it happens to the liquid service valve, it may be that the internal excess-flow safety valve snapped. Close the valve, wait a few minutes for the frost to dissipate, and reopen the valve.

If the converter/regulator is frosted, you have an engine-coolant flow problem. Assuming it's installed properly, it probably means low coolant level, a loose waterpump fanbelt, bad water pump, or other mechanical failure. Normally, the converter/regulator runs about as hot as the radiator.


If your engine ``runs out of fuel'', and you've checked the obvious (tank empty?) you can check for proper fuel flow right up to the carburetor, component by component. The following applies to Impco systems, but the same approach works for all manufacturers.

1. Disconnect the vacuum hose that goes to the filter/shutoff, at the manifold end.

2. Disconnect the vapor hose from the carburetor.

3. Suck on the vacuum hose. (This operates the shutoff, allowing LPG to enter the regulator/converter; you'll need to supply a constant source of vacuum to the filter/shutoff to keep it open for testing the fuel supply.) You should hear LPG vapor escaping the vapor hose. If vapor is available, the problem is in the carburetor or elsewhere in the engine.

4. If there is no vapor after step 3, and the filter/shutoff is connected to the regulator/converter with a hose (as opposed to a short pipe nipple), loosen the connector between the two as you suck on the vacuum hose. Liquid LPG should escape (keep your body out of the way). If it does, the regulator/converter is faulty. You can disassemble the regulator/converter and clean it; it contains only rubber diaphragms and a few springs.

5. If no liquid LPG is available, either the filter/shutoff is dead or the LPG hose from the tank is crushed or plugged. Turn off the liquid service valve on the tank, and disconnect the main supply hose at the filter/shutoff. Then momentarily open the liquid service valve; LPG should rush out of the hose into the atmosphere, quite noisily. If not, the hose is plugged. If it does, the filter/cutoff is suspect.


Only on long trips, when filling up, the fuel would enter the tank extremely slowly, less than a gallon a minute. At times, at high temperature especially, the problem became severe. I even took it to the LPG dealer I bought the parts from, and they were stumped as well.

Finally it failed, and hard. I had just driven 280 miles, about my range limit, to Farmington New Mexico. I went to fill up at a KOA Kampground, and the definitely empty tank would not take a single gallon of fuel! Long story short -- I managed to get to an actual LPG vehicle service station (Randall's Repair, see SUPPLIERS AND RESOURCES, and determined that the check valve on the fuel-filler was sticking. However the problem persisted after replacement. Randall then noticed that I had two check valves; one built into the tank (required) and one at the fuel-filler, where the hose is connected. There can be only one check valve -- when there's two, they fight each other under pressure and essentially don't open more than a crack. I replaced the filler-neck check valve with a straight-through filler -- problem disappeared.

I had to remove and replace one connector, where the hose from the tank fitted to the filter/shutoff. The brass fittings connecting the filter/shutoff to the regulator/converter leaked; I had to remove them and assemble with teflon pipe dope (which I should have used when I first assembled it).

I also had a textbook case of ignition troubles. On my first highway run, the engine started ``cutting out'' at high speeds, and when I slowed down to about 50, it smoothed out. Since the only new thing on the engine was the LPG system, I theorized various complicated fuel problems. When stumped, I called the dealer, who asked ``does it backfire through the carburetor?'' to which the answer was ``yes'', which always means -- ignition problem! But it's the same ignition I've had for years! No kidding -- and by wiggling and tugging ignition wires, I found the wire from the distributor to the electronic ignition was oily and poorly crimped!

I crimped on all new ring terminals, and soldered them all for good measure. I also found a crack on the ancient (ca. 1970) spark coil, and replaced it for good measure. No more problems! The moral is that LPG makes higher demands on the ignition than gasoline, but nothing that a decent system can't handle.


No one I talked to had ever heard of a ``gas can'' for LPG, something I thought was mandatory for a practical vehicle. Standard practice is apparently to tow an out-of-fuel vehicle to a filling station! I also wanted to be able to carry extra fuel for when I couldn't find a filling station on long trips (which has turned out to not be a problem).

The solution turned out to be easy. I bought a standard LPG container, and made up a special hose and adapter to fit it. One end is a standard POL tank connector, and the other is the special fuel filler connector, with about three feet of size #6 hose.

The gas can components; the 5-gallon tank on the right and the made-up hose (POL one end, Acme the other) on the left. In the foreground are two nice accessories; an adjustable wrench for the POL connector and some spare O-rings for the Acme connector.

Portable LPG containers are meant for dispensing vapor for camp stoves and the like; all instructions say to operate them in an upright position. This is because the vapor boils off the top of the liquid in the container, like a tea kettle, and tipping would cause liquid to flow out -- which is exactly what we want. Here's how to fill your car from your gas can:

Donna giving the ol' Rambler a fill up. Notice that the gas-can is held upside down.

1. Fill the portable LPG container normally.

2. Connect the hose to the portable LPG container.

3. Connect the fuel filler to the car.

4. Turn the portable container upside down.

5. Open the LPG container valve while holding the tank upside down. Close the valve when you've dispensed enough fuel, or the hissing (indicating fuel flowing from the can to your car) stops.

(When dumping gasoline into a car tank, gravity moves the fuel, and the air displaced by the liquid gasoline simply flows out the filler neck. Not so with LPG -- at a filling station, LPG is forced into the tank with a pump, with the vent open to detect a full tank.)

When you open the valve at step 5 above, the pressure in the gas-can will force LPG into the car's tank, until the pressure is the same in both tanks. Luckily -- the car tank is so much larger than the gas-can that nearly all the LPG will enter the car tank. With my 20 gallon car tank empty, and a 5 gallon gas-can containing 4.5 gallons (its maximum capacity), about 4 gallons of fuel are delivered into the car's tank.

For added safety and to prevent loss by leakage or the valve opening, I disconnect the adapter hose from the tank, storing it in a plastic bag to keep it clean, and install a brass plug in the tank, and tighten it. Even if the valve is opened, LPG will not escape. I keep a 5 gallon tank bungie-corded in the back of the car, with room for a second on long trips. I also use the same can for campsite-cooking!


Here's some things you might want to keep in the car:

A 10 inch adjustable or 7/8 inch open-end wrench, for the gas-can hose fitting. At the least, you'll need one to remove the gas-can hose when you fill the can.

Get a bunch of spare O-rings for the filler neck. They are cheap to replace, and will eventually wear out, and I had mine fall out once after a fill up. Without one, you won't be able to fill the tank!

Store the gas-can hose in a plastic bag to keep it clean.


Here's a breakdown of costs from my receipts for this project. I've arranged the tables with the major, not-optional items at the top. If you can scrounge hose and connectors and other useful doodads obviously you could save a lot of money.

The following items are essential, but probably scroungeable to some degree. You might also check truck junkyards, as many commercial and fleet vehicles use LPG. Used equipment is fine, as long as it is undamaged.

(Prices are from 1990)

Major Components 
Filler Valve Assembly           45.00 
filter/Shutoff, Impco VFF30     56.50 
Converter/Reg., Impco Model L  110.00 
Carburetor, Impco CA125         67.00 
CI-28 Air Filter Cover           9.44 
K&N Air Filter                  13.00 
Fuel tank, new (varies)        400.00  estimate
The LPG fuel tank is obviously a non-optional item. It is the single most expensive component, and the hardest to fit. The price of the LPG fuel tank will vary widely, depending on your application. Used tanks are a definite possibility.

These are definitely candidates for ``junk box'' items. The exact quantities will depend on your installation; for example, you could eliminate a foot or so of hose and the connectors and pipe fittings between the filter/shutoff and regulator\slash converter if you had the room to connect the two with a short pipe nipple.

Major Components
                             Quan     Cost
LPG fuel line, #6            14 ft     $2.54/ft      (tank to convertor)
LPG fuel line, #8             4 ft      3.32/ft      (tank to filler)
LPG fuel line, #4             4 ft      2.64/ft      (vapor bleed)
1" Vapor Hose                 2 ft      3.68/ft      (convertor to carb)
Vacuum Hose                   3 ft      1.00/ft
90 degree Elbow, #6           4         6.50 ea
45 degree Elbow, #6           1         1.28 ea
Straight Connector, #8        1         3.28 ea
Straight Connector, #4        2         1.84 ea
Vapor Hose Elbow              2         1.44 ea


Home Power Magazine, Box 130, Hornbrook CA 96044-0130 (916)-475-3179 Aptly subtitled ``The Hands-On Journal of Homemade Power''. Subscriptions available.

Real Goods Trading Co, 966 Mazoni St, Ukiah CA 95482 (800)-762-7325 (in CA (707)-468-9214) Their Alternative Energy Sourcebook really is ``A Comprehensive Catalog of the Finest Low-Voltage Technologies'', each section containing background technical information on batteries, power sources, lighting systems, etc.

Suburban Propane LPG components, systems and fuel. Check the Yellow Pages for one near you.

Solar Mind Magazine, 759 South State \#81, Ukiah CA 95482 (707)-468-0878 Issue #3 was on hydrogen vehicles and solar/hydrogen sources. Subscriptions available.

The Propane Directory, compiled by Liquefied Gas Directory of America, Inc., 2888 Highland Drive, Salt Lake City UT 84106 I have no further information at this time, even whether or not they still exist.

Folsom Auto Supply, 1048 Folsom St, San Francisco CA 94103 (415)-861-0800 Auto parts supply and machine shop service. They use water/alkali solvents in their hot-tanks instead of petro-based solvents.

Handbook of Chemistry and Physics, Chemical Rubber Company, 41st edition.

Pocket Ref, by Thomas J. Glover, Builders Booksource, 1817 4th St, Berkeley CA 94710 (415)-845-6874 An indispensible pocket version of the CRC book (sort of almost, but better) for modern times.

Addendum to WPS LPG automotive fuel conversion


This is the addendum to the original How-To...LPG booklet, written in 1990. A lot has happened since then, though amazingly what's in the booklet is still correct. This addendum covers some of the hard parts that followed, such as fuel efficiency.

This addendum contains followup technical, operational, and subjective data that I simply didn't have when I wrote the booklet -- obviously missing was fuel mileage data, long-term operational and tune up issues, reliability, etc. While these are not really adequately dealt with here, it's probably all I'll ever do on it.

The real problem with this whole project, from a documentability point of view, is that it wasn't simply a science project -- it was also my daily transportation, a fun toy, an art project (I'll cover that part of this car in a separate, much more-fun document, in the next few months XXX years). When I started this in 1987--1989, I was quite poor, using scrounged and occasionally inadequate parts for the rest of the car, necessitating doing some things twice. Everything done on this project during that period happened in extreme slow-motion, if at all.

The fundamental drive behind this project remains: if gaseous fuels are so good for internal combustion engines (mainly in terms of overall fuel efficiency and vastly reduced emissions) why isn't it common? In essence, is there a catch? The answer remains no -- there is no catch. It's industrial laziness, and lack of vision on a national level. And yes, this is not a final solution to any transportation problem, only one component in the existing system.

-- Tom Jennings, January, 2003


Fuel mileage on LPG for my car is, considered as a system, fairly good. A factory-stock 1963 Rambler Classic Wagon, outfitted with manual overdrive transmission and 196 ci. (3.2 liter) engine, got 19MPG best case on the highway, which works out to 6630 BTUs of energy per mile. Mine has a much-newer 232 ci. (3.8 liter) engine, a non-overdrive automatic transmission; total 1996 average fuel mileage was 14.22 miles per gallon LPG, about 6444 BTUs per mile, equivelant to 19.55 MPG gasoline. Considering my larger engine/less-efficient transmission, I'm netting a decent improvement over the best this car ever could do on gasoline.

(I started recording fuel consumption/mileage data in 1994; with a few gaps due to screwups on my part, I have a record of every LPG purchase, odometer reading, date and usually location since.)

(September 2002 -- a recent trip from Los Angeles to Santa Fe and back, with a lot of meandering, some 3000 miles, the mileage remains over 14 mpg LPG, with a peak (300+ miles) at 15.6 mpg LPG, approximately 21 mpg gasoline equiv.)

These improvements are due to not to a magical fuel, but to a vastly but simply improved fuel and ignition system. Hand in hand with improved mileage goes exhaust emissions; while I don't have complete data on before/after emissions, the emissions improvement is mostly due to LPG fuel:


Relative fuel usage, compared in BTU per mile. Assumes 60 mph, as mileage
is measured at more or less that speed on long trips; presumably BTU/mile
is lower at lower speeds due to wind resistance and internal friction.
BTU::fuel data from http://usapc.army.mil/alt_fuels/documents/Energy%20Equivalents%20of%20Various%20Fuels.pdf
LPG     Gal/    BTU/    equiv gasoline
MPG     Mile    /mile   MPG
11      0.091   7664    14.88
11.1    0.090   7595    15.01
11.2    0.089   7527    15.15
11.3    0.088   7460    15.28
11.4    0.088   7395    15.42
11.5    0.087   7330    15.55
11.6    0.086   7267    15.69
11.7    0.085   7205    15.82
11.8    0.085   7144    15.96
11.9    0.084   7084    16.09
12      0.083   7025    16.23
12.1    0.083   6967    16.36
12.2    0.082   6910    16.50
12.3    0.081   6854    16.63
12.4    0.081   6798    16.77
12.5    0.080   6744    16.90
12.6    0.079   6690    17.04
12.7    0.079   6638    17.17
12.8    0.078   6586    17.31
12.9    0.078   6535    17.44
13      0.077   6485    17.58
13.1    0.076   6435    17.72
13.2    0.076   6386    17.85
13.3    0.075   6338    17.99
13.4    0.075   6291    18.12
13.5    0.074   6244    18.26
13.6    0.074   6199    18.39
13.7    0.073   6153    18.53
13.8    0.072   6109    18.66
13.9    0.072   6065    18.80
14      0.071   6021    18.93
14.1    0.071   5979    19.07
14.2    0.070   5937    19.20
14.3    0.070   5895    19.34
14.4    0.069   5854    19.47
14.5    0.069   5814    19.61
14.6    0.068   5774    19.74
14.7    0.068   5735    19.88
14.8    0.068   5696    20.01
14.9    0.067   5658    20.15
15      0.067   5620    20.28
15.1    0.066   5583    20.42
15.2    0.066   5546    20.56
15.3    0.065   5510    20.69
15.4    0.065   5474    20.83
15.5    0.065   5439    20.96
15.6    0.064   5404    21.10
15.7    0.064   5369    21.23
15.8    0.063   5335    21.37


Until recently, the biggest annoyance of using this car as a daily-driver was due to the combined factors of fuel availability, vehicle range, and tank-filling ambiguity. The problem wasn't miles per gallon, per se, but the real-life vagaries of availability.

To illustrate: in a gasoline car, in general around-town driving, you might drive until you know you have only a few gallons left, then fill up at your leisure at one of the plentiful 24-hour filling stations. You might put off filling up until the next day if you're in a hurry.

For example, you have a 14 gallon fuel tank, and get 20 MPG, and stop at a filling station when you have two gallons remaining; when you add fuel to the tank, you easily get within 1 gallon of full. These factors combine to give you an average-worst-case practical range of (11 gal*20mpg=) 220 miles between fillups.

I originally built this car with an 18 gallon LPG tank (at 80%), and get 14 MPG, with decent efficiency, as noted above. Now most of my filling-stations are closed after 5pm and on Sundays; in general I don't like being under 1/4th full, which is about 4 gallons (though I'll certainly run to the grocery store without worry). When I add fuel, or more accurately, when the poorly trained, slightly fearful low-level employee at the LPG filling station pumps fuel, I frequently do not get a truly full tank; I can't count on having more than 16 gallons in my 18 gallon tank. These factors combine to give me an average-worst-case practical range of (12 gal*14mpg=) 168 miles between fillups. If I don't fill up Saturday, I can't do much driving on Sunday without a search for fuel.

(The above applies mostly to local driving; on long road trips, the distances covered generally means you find LPG pretty much anywhere, though lessened on Sundays.)

The solution to this is obvious -- since I can't change availability, increase tank size! In February 1997, I installed a 35 gallon/28 gallon liquid capacity tank that increases my effective range to 300-350 miles, which utterly transformed the car -- I buy fuel every few weeks, and on long road trips, buy fuel once per day. For local driving, I sometimes drive to a dealer in Gardena (from Hollywood) and fill up -- it's worth it when you're buying 20 gallons at $0.81 per gallon. If I'm lazy I pay $1.40 per gallon at a tool rental place three blocks away.

I can't emphasize how much this has improved the car. The downside was, I had to gut the trunk space under the rear of the station wagon to mount the tank; until I find or make a "continental kit" for the spare tire it sits on the roof (ugly). As mentioned in the LPG Book, commercial LPG tanks come in standardized sizes and shapes, and are therefore not contoured to the chassis like OEM gas tanks.

Take this as a BIG HINT -- you want absolute maximum fuel capacity for a daily driving LPG car, I would suggest my rule of thumb, a real 300 mile range that leaves you with a few gallons for peace of mind. And you must do this safely -- no tanks or lines in the passenger compartments, etc -- with off-the-shelf parts.


I've gone through too many iterations of LPG hardware to get where I am. A lot of it is certainly my own inexperience, but a too-large portion of it is incredibly poor advice from so-called experts and a complete lack of interest and documentation from equipment manufacturers.

I'll skip all the excruciating detail. Suffice to say I have a nice normal inline-six of 232 cubic inch capacity (about 3.8 liters) that produces only about 90 horsepower but tons of nice twisty torque. The engine is now dead-stock except as follows:

        Impco model CA125 LPG carburetor
        Impco model 'L' LPG convertor
        Autotronics MSD-5 electronic ignition
        Autotronics 4046 Close-Loop Oxygen Controller
        1979 AMC distributor/electronic ignition
        Two-wire oxygen sensor in tailpipe

This stuff is not expensive. The CA125 carburetor is probably the most common LPG carburetor, has two moving parts, and requires a $50 tuneup every 50,000 to 100,000 miles. Mine was new in 1989 and has been tuned up (replace diaphragm) once, in Jan 1996. (Impco's patents ran out on the CA125, and it is now cloned; in fall of 2001 I bought a brand new Nolff CA125 mixer for $59.00, less then the cost of the Imco brand rebuild kit!) Before I installed the 4046 Oxygen controller, in 1995, I constantly adjusted both low speed and high speed mixtures trying to optimize mileage, to no avail. Since the 4046 was installed, and carburetor adjusted, very little adjustment has been required; these were small adjustments to put the closed-loop in the center of it's range (see raw notes, above) or to change idle speed for subjective 'feel' improvements. Please see the carburetor selection nomograph to select the right carburetor -- you would-be hot-rodders, don't try to select a larger carburetor, they don't work like venturi-based gasoline carbs.

The Model L convertor was also purchased in 1989 and tuned up (diaphragms, valves replaced), also in 1996. It has no adjustments.

The MSD-5 ignition I purchased in 1985 for another car; I simply installed it on this car, originally on the old mechanical-points distributor, then later on the replacement 1979 distributor. It has no adjustments of any kind.

The 4046 O2 controller required one go-round of carburetor low- and high-speed mixture adjustments at installation time. The 4046 dynamically sets the fuel/air ratio at the carburetor, so the manual carburetor adjustments consist of setting the carburetor slightly rich so that the 4046 can lean it out on-the-fly by measuring tailpipe oxygen content. (It applies a modulated engine vacuum signal to the atmospheric vent on the regulator; it can lean the mixture but not richen it, hence the static setting is rich.)

(September 2002: I'm embarrassed to say I just fixed a major problem with the installation of the 4046. The 4046 modulates a vacuum source applied to the convertor to vary the mixture; I used manifold vacuum. WRONG! It *must* be picked off from a weaker source, such as the recommended mixer, below the diaphragm, above the throttle butterfly. The installation sheets are very vague about this -- and it's critical. It worked on manifold vacuum, though not well; symptoms were motorboating (hunting) as the slighest "on" pulsing of the 4046's solenoid would radically lean the mixture; the weaker vacuum source slows the loop response.)

The 1979 AMC distributor was chosen for a number of reasons, after a lot of agonizing "research". There seems to be very little acknowledged "science" in setting ignition timing curves; suffice to say here, the 1979 distributor had a much more agressive advance curve than the original distributor, which is basically what I was after. I plotted measured distributor advance curves by hand. It also had the advantage of being all-electronic, eliminating annoying and hard to adjust points, and all but eliminating shaft wear due to points-spring tension (a major source of hard-to-tune older engines, by the way!).


To be blunt, I now consider a closed-loop fuel system, on any car of any kind, to be entirely non-optional. I am now quite convinced that all older carbureted, open-loop fuel systems have essentially the same problem I had with mine, regardless of fuel, but most people are not measuring just how awful it really is.

Without the O2 controller, I simply could not get an optimum mileage/performance configuration. I'd end up with good low-end performance, but high-speed fuel consumption was terrible, or vice-versa. Most of my research was done on the road, through detailed and copious notes, but I also paid for four hours on a road dynamometer in 1991 which resulted in little new data (essentially I found less-non-optimum settings for the see-sawing adjustments).

The closed-loop system simply solved all problems. I had a few gyrations to go initially with optioning the carburetor, see notes, below. Essentially, you tune slightly rich with the loop open, eg. solenoid disconnected etc; the 4046 leans the mixture when the O2 sensor in the exhaust says it's rich. drops). With these settings, running open-loop, low-speed performance is reasonably OK, high-speed performance OK but mileage not great due to the tendency to run rich on throttle-up and hard accelleration.

Closing the loop, by enabling the 4046, causes the Controller to measure oxygen content at the tailpipe, through the oxygen sensor installed there, and modulate the fuel/air ratio produced by the carburetor by pulse-width-modulating a vacuum signal applied to the LPG Convertor. A small solenoid quietly buzzes as it dynamically adjusts the vacuum signal; measuring the average voltage across this solenoid is a direct indication of the loop status; 5-10V means the system is able to maintain the loop, with higher voltages indicating that it's having to apply more vaccuum to lessen the fuel to lean out the mixture; this is called the "error voltage" because it is directly proportional to how much the controller has to change the mixture (eg. how far is is from perfect.)

While a decent description of closed-loop servo systems doesn't seem appropriate here, you should note that the carburetor isn't controlled with a smoothly-varying adjustment provided by the Controller; instead, it pulses rich/lean/rich/lean... dozens of times per second, and measures the average result at the tailpipe. This works just fine because the motor is a big, slow hunk of iron, and the fuel/air mixture moving into the cylinders is also big and slow, relative to the nimble rich/lean adjustments; the big/slowness averages out the pulses into an effectively smooth mixture. You will notice that the average voltage across the solenoid varies slowly up and down somewhat; this is called "hunting", and a small amount is quite normal. If the engine speed "motorboats" fast and slow you've got a problem; this is covered in the Autotronics documentation. The varying of rich/lean/rich/lean... is called Pulse Width Modulation.

[The oxygen sensor by itself is also useful as a tuneup indicator. Please note that it is decidedly non-linear; it CANNOT IN ANY WAY be coerced into giving you a direct percent-oxygen measurement! It measures one specific thing: the presence/absence of oxygen on the hot side of the sensor, ie. inside the tailpipe, relative to the cold (outside) side. I installed the O2 sensor a few months before the 4046, and was able to closely set low speed mixture by enriching until the sensor indicated no O2, then leaning out til O2 is indicated.]

One device I found to be an utter waste of time and money was the Autotronics 4052 LPG Ignition Recurve Computer. It essentially recalculates a new ignition curve based solely upon engine speed, and while it might be useful for very heavy vehicles it generally just caused pinging on my car. It's a pain to adjust too.



The secondary effects in this area are all good. As is well known, reciprocating engine piston rings are far from a perfect seal; some fuel/air mixture always leaks past them, and worsens with age. While this is true for LPG as well as gasoline, when the cylinder walls are relatively cold, liquid fuels condense on the sylinder walls and wash the oil film off, ruining lubrication and vastly increasing wear. The liquid gasoline carries with it unburnt fuel components (most visibly black carbon) which ends up in the engine oil -- a quick glance at the dipstick on any engine will show you the result. New engine oil, a dull yellow color when first installed, becomes predominantly grey/black after 1000 miles or less.

While gaseous fuels also leak past the rings into the crankcase, they do not wash oil from cylinder walls, and except for extreme age or mechanical trouble don't generate bulk carbon, and hence the lubricating oil stays very nearly uncontaminated. Because they are gaseous, they are easily recirculated into the engine via the PCV system. (Probably some trace amount of fuel is also dissolved in the oil.)

Fact: my engine now has over 110,000 miles [Nov 98], consumes no (zero) oil, and engine oil stays bright yellow after 4000 miles (when I change it). I have gone as far as 8000 miles between changes, and the oil only darkens slightly. (September 2002 note: at 180,000 miles there is a little seeping from the rear seal; now at 3000 miles the oil is a greyish yellow.) It seems to need the valves rebuilt, based upon comparative annual cylinder-compression data from 1991; as mentioned in the original edition, this engine doesn't have hard valves, and the squirt-of-oil test seems to indicate the dropping compression (115-125, from 140-145 in 1991 [140-130 in 2000]) is valve wear.

Oil needs to be changed regularly anyways, as the molecules sustain shear damage and chemical changes due to heat, acid buildup from unburnt fuel, impurities, etc.


In 1987 rebuilt my engine, from a bare machined engine block on up, in the standard manner, and ran it on gasoline for two years/27,000 miles. In 1989 I installed this engine in the curent car, 87,000 miles on the speedometer, at which point it was converted to LPG fuel. It now has 170,000 total miles on it [Nov98], 90,000 on LPG. It does not seep or drip engine oil from any gaskets (except an annoying valve-cover gasket that I poorly sealed). I believe this to be due to the lack of gasoline and its combustion by-products, a very active solvent, in the engine oil. I certainly didn't do anything extrordinary when I assembled the engine.

One 'undesirable' (sic) effect is that the underchassis of my car has rusted, because the engine oil that would normally spray and drip under the engine is not "undercoating" the chassis. If you think I'm exaggerating, casually examine a half dozen or so gasoline cars over 10 years old and you'll see that their undersides are coated with a sticky black, tarry substance made of engine oil and road dust. Engine oil drips and mist is also the source of the darker section or dark spots down the center of the driving lane on roads.


These are random notes from my files over the years, in no particular order.
1995: One on-road failure: whiel cruising at 60MPH, car suddenly acted
fuel-starved (depressed throttle, engine would 'bog', let up, depress
only slighlty, engine cruised OK; could not maintain speed). Pulling
over to the side of the road and lifting hood revealed LPG convertor
had turned into a giant snowball -- water flow ceased. I unbolted it
from inner fender (two 1/4-20 bolts), removed water access cover, about
100mL of sludge fell out. Convertor, mounted at slight angle on inner
fender, had accumulated sludge from cooling system, eventually plugging
flow. Dumping out solved on-road problem. Months later [it took a few
years to plug up, no hurry here] moved convertor, made level.
Jan 96: Convertor rebuild. Wear was reasonable; stretched diaphragms, wear
on valve faces, etc. Wax buildup. Cleaned with Dr. Bronners peppermint
soap. Rebuild took 30 minutes.
Convertor runs fine on it's side; however wax will build up instead
of running out the gas outlet.  Is there a simple way to drain it?
How often?  With convertor lower than carburetor, wax did puddle
insignifigantly (2-3 mm) in the bottom of the vapor hose nipple on the
convertor after many months of operation. [September 2002: this
is a false worry. Ignore it.]
CA125 throttle ass'y not symmetrical; cannot be rotated 90
degrees! However adapter Impco #A3-32 mates a CA125 mixer to a T2-4
throttle body (fits CA225) which is symetrical.
The CA125 has had the lean air dam assembly (AV1-14-4) installed since
July 1993. It adjusted reasonably well for idle and high speed operation
prior to Oxygen Computer installation.  With the 4046 installed, it was
difficult to get it adjusted for both idle and high speed; idle was fairly
simple, but high speed required +2 notches rich (R) on the mixer assembly,
derived by cruising at speed (60mph), accelleration/decelleration, etc,
and adjusting the mixture control until the error voltage stayed within
range. It tended to run generally too lean, and the error voltage was
erratic.  When I installed the rich airdam assembly (AV1-1447-2), I set
the mixer mixture to centered (0), and assumed I would have to repeat the
above process. The idle required only 1/4th turn leaner with AV1-1447-2
vs. AV1-14-4. When I set out to road test, I was surprised to find the
4046 error voltage stayed in range with the default mixture setting (0) --
under all conditions, including heavy accelleration (50mph up to 75mph),
except for lean-out on high speed throttle-up (same as before).
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