Propane Trick - the original post
Propane Trick -- by Rich Motts aka RRich
Often engine ills can be traced back to the fuel mixture, too much or not enough fuel. Measurements can be done with expensive instrumentation to determine whether the mixture is right for that engine, but there's an easy and inexpensive way as well. Propane burns nearly the same as gasoline, and can be used to temporarily richen the mixture.
Use an inexpensive propane torch, the cheap ones are the best. It takes a little modification to the torch before it will work effectively. Remove the tiny orifice, the tiny hole that restricts the amount of propane that can be released. Sometimes the orifice is removable, sometimes it needs to be drilled out. Use the smallest drill bit you have to enlarge the hair-like hole.
It will not do the job if you do not drill or remove the orifice, it will not give enough propane.
Also remove the torch head, either by unscrewing it or cutting it off. What you end up with is just the valve. Connect the valve to a small propane bottle, like the torch would use. To make sure you have correctly removed the orifice, hold the bottle with the valve down and open the valve - you should get lots of propane, some liquid, mostly gas. DO NOT DO THIS NEAR A FLAME - or you'll be surprised.
Slip a long vacuum hose over the pipe, long enough to reach the air intake stream, like the carb's snout, from the driver's seat. Now you have a tool that will help you diagnose lots of problems.
Operation - hold the bottle with the valve on top, you get propane gas for low RPM tests. Hold it upside down you get liquid propane for high RPM or under load tests.
BE CAREFUL - PROPANE IGNITES VERY EASILY -- Do not spray it on the distributor or on bad ignition wires, or on any sparking device, like an alternator.
Vacuum leaks - Valve at the top for gas - simply spray a little propane around the suspected areas, like carb base gaskets, vacuum tees, intake gaskets, injector seals, brake booster, hoses, etc. When you hit the leak with the propane the RPM will change. If someone has already tried to adjust the carb with the leak present, the carb may well be adjusted too rich, nullifying the propane. In that case, simply lean the mixture down with the idle screws till it runs rough, then use the propane. Make it lean on purpose and the system becomes very sensitive. Even tiny leaks can be found in this manner. Unlike liquids, like carb cleaner, gasoline, ether etc., propane will not wash dirt into the leak's hole, temporarily plugging it. And, unlike liquids, in case it does happen to ignite, it flashes, burns fast, scares hell out of you, but once it flashes the flame goes out. A liquid makes puddles that continue to burn causing extensive damage.
Yes, it will scare hell out of you - you'll leave some hair on the hood latch too.
Remember, propane is heavy, so it will collect in low places. It also tends to blow around, and being invisible you can't see where it's really going. Use it sparingly. -- (No, the heater hose doesn't have a vacuum leak, the propane's blowing back into the carb.)
Idle Mixture - Valve at the top for gas - if the mixture is set properly, adding just a touch of propane will speed the engine up ever so slightly. Adding more propane will cause the speed to drop. Experiment with that - notice just a little propane speeds it up, more slows it down. That's because the ideal setting is just slightly lean. If it was set too lean. the R's will really rise - the engine "likes it." If it was too rich, adding propane will make the R's only drop. The curve is a bell shaped curve, with a slightly flat top. Get a good "feel" for that.
Cruise Speed under load - Valve at the bottom for liquid. Tape or attach the hose to the air intake - right in the airstream. Run the long hose and valve to the driver's seat. Go for a ride. The same principle applies at speed under light load. Adding propane should just barely increase the speed if it is already getting the right mixture of gasoline. Adding more should slow it down - too rich. If the engine "really likes it" you were too lean. This is a great way to determine whether or not you need to change jets, and which way, richer or leaner. Who needs $50,000 worth of equipment when you can do it for $5?
Diagnosis - often the question is whether or not a problem is fuel or ignition related. Simply duplicate the condition where the poor running or misfire occurs, then add propane. Obviously if adding the fuel makes the problem "go away" it was a fuel problem.
Stumble On Acceleration - a popular problem. Bad accelerator pump? Snap the throttle to make it stumble, but while you snap the throttle, snap the propane valve open at the same time. It takes a little practice, but if the stumble disappears -- guess what? Propane won't help wrong timing advance.
Closed Loop Systems - Whenever you add propane the engine will react as above, but the computer will see the RPM change and compensate in 2 ways. At idle it will re-adjust the IAC till the RPM returns to the preset idle speed. At higher speeds the O2 sensor detects the added fuel and cuts back on the gasoline. The end result is it will momentarily have a reaction, then return to what it was before. Then, when you turn off the propane it will do exactly the opposite. For example on an engine with a vacuum leak: The engine will be running rough but at normal speed. When you "hit the hole" the R's will increase a moment, then drop down again. But - you'll notice the engine is running smoother now. Then when you remove the propane, the R's drop suddenly as it becomes way too lean for an instant, then it returns to normal again. The reaction, lean/rich, is the same as with open loop, but only for a quick instant.
"False Air" on MAF Vehicles - Spray a little on the ducting between the MAF sensor and the throttle body. The ducting should be sealed, no leaks, a common problem.
Priming an Engine - shoot a good shot of propane into the air cleaner, start it up. If it backfires it won't sit there and burn like it will when you pour gas down the throat - and you don't have to remove the air cleaner.
Most vehicles can run completely on the propane from your valve. If an engine is hard to start, try adding a shot of propane to see if it helps. Remember the amount of fuel is determined by the valve, the amount of air is determined by the throttle opening. I've even driven my van a few blocks on the propane when I ran out of gas in the rain and didn't want to walk.
You'll find this tool becomes one of your handiest tools in your toolbox.
Remember - propane ignites easily - observe all the safety rules, be careful and it can be your friend.
---- BE CAREFUL! ----
Last edited by Jim_Lou; 12-10-2007 at 10:00 AM.
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