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> JimA=> If your bus does this, then I would GUESS that something is > => set a bit rich, ... > > 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. What I wonder is how the leak contributes to the > popping. Or is it that the popping is normal and, since the leak is > before the muffler, I can hear it where I wouldn't if the exhaust were > tight? 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 FI will largely compensate for altitude changes, so the gas/air mixture should be about the same at your altitude as it is at sea level. I think you will find that your altitude has little to do with the backfiring; it will, however, reduce your power output as it will with any normally aspirated engine. 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 proceedure that never works. It has no chance of working because it leaves the engine in a state of continually drawing in an unmetered amount of air which can't be compensated for. It is easy to replace the pushrod tube seals, I probably even have some in stock. It is harder to get the factory calibration screw back close to the right place. Jim - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA *******************************