Those backfires probably aren't a problem with your exhaust system. Your engine is probably "dieseling".

Dear Car Talk

Dear Car Talk | Mar 01, 2000

Dear Tom and Ray:

I own a 1965 Chevy Corvair Monza. It has a 110-horsepower pancake
six-cylinder engine. The problem is with the exhaust system, I think. The
car has around 70,000 miles on it, and after I turn off the engine, about
five seconds later, the muffler emits a loud backfire that sounds exactly
like a gunshot. I'm not thrilled about that, but the bigger problem is that
the mufflers keep burning out (I get the cheap ones at Midas). I tried
replacing both carburetors, and that didn't help. Can these mufflers just
not take the heat of an air-cooled engine? I can get a better muffler (it's
around $100), but I want to know if that will solve the problem.
Supposedly, it's a muffler that was designed for the Corvair and can take
the heat of the exhaust. Should I get the expensive muffler? -- Kevin

RAY: No, you shouldn't, Kevin. I wouldn't get an expensive ANYTHING for
this car, to tell you the truth. Except maybe an expensive football helmet.

TOM: I don't think the muffler is your problem. I think the problem is
"dieseling," or "run-on." These cars are famous for it. They run so hot
that even after the ignition is turned off, it's possible for the
combustion process to continue -- sometimes for many seconds!

RAY: A friend of mine had a '65 Corvair, and we kept turning the idle speed
down ... and down ... and down in an effort to get it to stop dieseling. We
turned it down to half of what it should be and still, every time he shut
off the car, it would keep sputtering for about 15 or 20 seconds -- "budup
bu bup bup bup, budup bup bup." Of course, we were living in Texas, where
it was 108 degrees in the shade, so that didn't help.

TOM: When the engine sputters like that, gasoline continues to get sucked
into the cylinders. Some of it burns, but most of it does not. And when the
unburned gas finds its way into the exhaust system and reaches a critical
mass in the hot muffler -- KABOOM!

RAY: Run-on is usually caused by an idle speed that's too high, engine
operating temperature that's too high, timing that's too advanced or any
combination of the three. So fix that first, Kevin, or you'll keep going
through mufflers like my brother goes through ... uh, name something you go
through quickly, Tommy ...

TOM: Spouses.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One