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My comments below. Toby Erkson email@example.com <-- Please use this address for email '72 VW Squareback 1.6L bored and stroked to 2.0L, Berg five-speed '75 Porsche 914 1.8L, ORPCA member Portland, Oregon, http://www.geocities.com/MotorCity/8501/ >-----Original Message----- >>...Sorry, I should have been more explicit. The bus has a >fuel-injected 2L T4 >> engine and an obvious exhaust leak just before the muffler, plus it >> ordinarily operates at over 5300 feet altitude. So yes, lowish oxygen >> content makes for a relatively rich mixture, and there are >lots of sooty >> tailpipes up here... > >The most common cause of backfires is an engine that is drawing in a >mixture that is too lean to burn, so it gets pumped on through into >the exhaust where the gasoline can accumulate. I'm going to guess >that the FI in your bus is the L-Jetronix. The 1.8L used L-Jetronic, the 1.7L and 2.0L used D-Jetronic. The L-jet system is actually better than the D-jet but it is more sensitive to vacuum leaks. I do know that the L-jet does not squirt the injectors when the accelerator pedal is not being used (like coasting down a hill). Like Jim said, the FI compensates for the altitude so your engine shouldn't be running rich -- suspect a leaky injector or a poorly functioning cold start system and make sure all vacuum lines are tight! >This system has the very common problem that it develops air leaks >into the crankcase (most important are the pushrod tube seals) which >make the engine run lean. This is usually treated by those who don't >understand the problem by removing the sealed factory calibration >plug and attempting to readjust the mixture, a procedure that never >works. Uhm, not quite sure if you are referring to the L-jet system, but the plug that it sounds like you are describing is the CO2 screw on the air meter. Yes, there is NO need to mess with it. Same goes for the fuel pressure can.