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