Engine backfire

Too much oxygen in exhaust, letting unburned HCs ignite prior to catalysis.

Bad EGR, O2 sensor, hole in exhaust system prior to catalytic convertor, or maybe other bits & pieces of anti-pollution crap!

Had a REAL problem with this in my '84 Isuzu Trooper II 1.9L 4-spd. If it hadn't eventually been fixed under 50k pollution crap warranty, I would have taken a wrench to someone's noggin! Three very frustrating, indecisive trips to dealer required, followed by a nastygram to dealership owner finally got it straight!

Best of luck!

Paul B.

01 XJ

This is what Chris (c) wrote in one of my threads regarding a leak down test. Note the reference to an occasional popping:

"It is most common for racers to use leakdown to test the condition of their engine, but there is much more you can diagnose with it. An engine can show relatively good compression numbers and still have a problem, but not necessarily all the time. For instance, a head gasket that has started to seep (and will fail eventually) doesn't always get detected with a compression test due to the short time the cylinder sees pressure while cranking the engine. A leakdown test will find that seeping gasket if the air is left on long enough. I have also detected a bad exhaust valve with a leakdown that wasn't detected with a compression test. This particular engine ran fine, except for an very infrequent pop out the exhaust pipe. The leakdown showed it was only holding 40% as compared to 80-85 for the other cylinders. When we tore the heads off, there was an exhaust valve with a small crack from the stem out to the valve seat. I would bet that eventually this valve would have broke and caused some serious damage. It was on a Dodge 440 that got driven pretty hard."

Hope this helps,

Bill

You may have a gap or hole in the pipe before the oxygen sensor. It will eventually burn out your cat converter also and you'll have to replace it to make it through emmisions. Nick

Could be: Exhaust leak Burnt or leaky exhaust valve Retarded ignition timing and/or rich fuel mixture Some engines also have a dashpot type device to prevent the throttle from slamming shut when decelerating. This will also prevent this if the other conditions don't exist.

Chris

What engine and what Jeep?

Some GM engines have quirks....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

" snipped-for-privacy@msn.com" wrote:

Yick. Tracing out smog and exhaust leaks are way beyond my skill set. (in other words, sounds like I'm in for a pricey repair.)

What is the downside to leaving it unrepaired? (Emissions are not a concern as I have antqiue tags and am therefore inspection exempt.)

Also... it is virtually always sounds like it is the right exhaust pipe, not the left. Does this indicate that my air leak is probably in that half of the system?

Not sure I follow. If by "cold" you mean with the engine not running... there wouldn't be any sound anywhere in the system... so I'm not sure how I would find the leak with a stethoscope?

Correct.... and it is a CJ7 jeep, but virtually none of the mechanical or electronic components are the original AMC manufacture, so don't read too much into that. Basically a CJ7 body with all kinds of Ford, Chevy, Dodge, and aftermarket parts underneath! (no honda parts as of yet... ;-)

So I take it it is a 'built' 350 or is it a direct transplant?

Ok, anyway I have two thoughts....

One is it has been built and the intake manifold needs to be retorqued down because it is leaking allowing a lean backfire.

This can be found by using a can of carb cleaner spray. At idle spray along the gasket seam especially at the 4 corners. If you have a leak, the idle will burp. A cracked big vacuum line like the PCV or brake booster line can cause it too, so spray the vacuum connections as well.

Second is the exhaust manifolds need to be retorqued down. If they were messed with and not just transplanted, then they likely need a second torque like the intake. These tend to just plain come loose.

Then a third is maybe the bolts at the exhaust flange has worked loose.

The exhaust leaks can be located by using a piece of tubing or even a chunk of garden hose held to the ear. If there is a leak you can hear it through a piece of tube held close to it.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's