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Re: Backfire

> 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 

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 Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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