Back firing is always the result of unburned fuel or air getting into the
exhaust system. This can be from bad throttle settings to a problem with the
deceleration valve, (is yours even there) to leaking exhaust or often with
the air cooled engines, loose heads or burned up seal rings, (gaskets), at
the top of the cylinders.

Checking the exhaust is easy. Plug the tail pipe with rags or a potato or
someting while the engine is idling. A healthy exhasut will stall the engine
when blocked.

Note that a loose head may also cause this test to fail.

Next step is compression and leak downt test to look for the loose head or
bad valve.

Does the engine get louder, like exhaust louder under load? If soo, loose
heads are likely.

Dennis

>From: vanagonvw <vanagonvw@GMAIL.COM>
>Reply-To: vanagonvw <vanagonvw@GMAIL.COM>
>To: vanagon@GERRY.VANAGON.COM
>Subject: Backfiring. How to proceed?
>Date: Thu, 31 May 2007 11:05:09 -0600
>
>I have an 81 gas engine vanagon.  For 20 years of mt passes, on a rare
>occasion, it would backfire coming down a steep pass in 4th gear, but
>nothing to think much about.
>
>Recently, I had to replace the muffler, under the stress deadeline of
>passing the damn state tests, and my choice based on time and finances
>was a straight through, 'glass pak' steel  muffler. Please don't berate
>me for that I did what I had to do. :-)
>
>This was done in late winter, when  I drove with the windows up, and the
>stereo on, but as it has warmed, the windows are open, and I am hearing
>some serious backfiring, in all gears, when I back off, or go down even
>a slight incline. Its bad, really bad. I can assume it came around at
>the time of the new muffler, but since I didn't listen for it at the
>time, I am not sure.
>
>I think maybe it could have been  backfiring for a long time, but the
>old muffler was doing a better job of 'eating it' as opposed to this
>different one I have installed..... Either way, I need to fix it, so I
>can drive it.
>
>I have been told that backfiring is almost always a hole in the exhaust
>so that is where I am looking.
>
>All that was done, was to cut the pipe after the  flange,  weld the
>muffler on, and weld a strap to hang if from the crossover pipe. I have
>searched for signs of a leak, and cannot find any. Up close and personal
>to the muffler with it running, reveals nothing that I can see or hear.
>
>How does one actually test the exhaust system for leaks? The tin makes
>it inconvenient to check the pipes to the heads, so I haven't been there
>yet, but at such high mileage, I would be stunned to find that one of
>the bolts had come loose....
>
>I was thinking of using a compressor, like in a leak down test, only
>with the exhaust valve open, and then blocking off the tailpipe to see
>if I could locate any kind of hole.....Not sure if that is a good idea
>or not. FWIW, it does pretty well kill the engine if I block off the
>tailpipe while its running. That is how they test for leaks during an
>emissions test. Not very reliable, but I did it anyway.
>
>Just curious how others might proceed to locate the problem, and to ask
>if perhaps the problem could be elsewhere? If I had done anything to the
>engine settings, I could see that maybe it has leaned out or something,
>but nothing has been touched in that area for years.
>
>Much obliged for any ideas or suggestions, short of getting after me for
>putting on a cheap muffler :-)
>
>Thanks,
>
>John