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Car Forum / Jeep / February 2004Engine backfire
What are the most common causes and remedies for engine backfire? (At least, I think backfire is the correct term for my problem... it is an occasional loud 'bang' as if from gunfire, which seems to emanate from near the catalytic converters under my floorboard rather than up in the engine compartment.) Anyway, assuming that I am using the correct term... I have a jeep with a fuel injected chevy engine, and it backfires from time to time. Every occurence is when the engine is revving down (e.g., I put the clutch in.) However, beyond that, there is no rhyme or reason... sometimes it will happen 3 times in an hour, other times it might go weeks with no backfire. I can't detect any other common factor besides revving down (e.g., it happens regardless of whether the engine is hot or cold, what gas I'm running, etc.) >What are the most common causes and remedies for 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 > What are the most common causes and remedies for engine backfire? > [quoted text clipped - 11 lines] > besides revving down (e.g., it happens regardless of whether the > engine is hot or cold, what gas I'm running, etc.) 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 > What are the most common causes and remedies for engine backfire? > [quoted text clipped - 11 lines] > besides revving down (e.g., it happens regardless of whether the > engine is hot or cold, what gas I'm running, etc.) As Paul said you have oxygen in the exhaust, common on SMOG engines with an air pump, but an injected engine shouldn't have one to pass SMOG. Do you have one anyway carried over from an old AMC engine? Or you have an exhaust header leak that needs to be fixed. God Bless America, ßill O|||||||O mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > What are the most common causes and remedies for engine backfire? > [quoted text clipped - 11 lines] > besides revving down (e.g., it happens regardless of whether the > engine is hot or cold, what gas I'm running, etc.) 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? L.W.(ßill) Hughes III <billhughes@cox.net> wrote in message news:<4032CD9E.F2394E1@cox.net>... > As Paul said you have oxygen in the exhaust, common on SMOG engines > with an air pump, but an injected engine shouldn't have one to pass [quoted text clipped - 18 lines] > > besides revving down (e.g., it happens regardless of whether the > > engine is hot or cold, what gas I'm running, etc.) Make yourself a stethoscope and see if you can find it cold. Leaving it leak will foul your oxygen sensor, making your electronic injection richer, which in turn fouls your spark plugs and washes down your cylinder walls of lubricant, hastening wear. God Bless America, ßill O|||||||O mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > 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.) [quoted text clipped - 5 lines] > pipe, not the left. Does this indicate that my air leak is probably > in that half of the system? Just wondering - wouldn't the old test of hanging a shop cloth over the end of the tailpipe pretty well identify a possible valve problem? Hold it so it hangs straight over the end of the pipe. It will blow out, maybe falling back to touch the pipe, unless you have an actual exhaust valve problem which will try and suck it up the pipe when that cylinder is in the right position. That will cause a backfire, BTW, and it can be pretty hard to spot if it's a weak valve spring. On Thu, 19 Feb 2004 02:36:46 UTC L.W.(ßill) Hughes III <billhughes@cox.net> wrote: > Make yourself a stethoscope and see if you can find it cold. > Leaving it leak will foul your oxygen sensor, making your electronic [quoted text clipped - 12 lines] > > pipe, not the left. Does this indicate that my air leak is probably > > in that half of the system?  SignatureWill Honea <whonea@codenet.net> A bad valve does give an inconstant back pressure sound that reminds me of an old Crist Craft motor boat. God Bless America, ßill O|||||||O mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > Just wondering - wouldn't the old test of hanging a shop cloth over > the end of the tailpipe pretty well identify a possible valve problem? [quoted text clipped - 3 lines] > cylinder is in the right position. That will cause a backfire, BTW, > and it can be pretty hard to spot if it's a weak valve spring. > Make yourself a stethoscope and see if you can find it cold. 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? A crack in the exhaust system will be less likely to be found after the metals have expanded with heat. You don't need a stethoscope, you could put your hear directly on the manifolds if you like. God Bless America, ßill O|||||||O mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > > Make yourself a stethoscope and see if you can find it cold. > > 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? > What are the most common causes and remedies for engine backfire? > [quoted text clipped - 11 lines] > besides revving down (e.g., it happens regardless of whether the > engine is hot or cold, what gas I'm running, etc.) 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 > What are the most common causes and remedies for engine backfire? > [quoted text clipped - 11 lines] > besides revving down (e.g., it happens regardless of whether the > engine is hot or cold, what gas I'm running, etc.) OP said "injected chevy" I think you could bet the farm that it's a stock 350". God Bless America, ßill O|||||||O mailto:LWHughes3rd@aol.com http://www.billhughes.com/ > What engine and what Jeep? > [quoted text clipped - 3 lines] > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's > OP said "injected chevy" I think you could bet the farm that it's a > stock 350". 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... ;-) > > OP said "injected chevy" I think you could bet the farm that it's a > > stock 350". [quoted text clipped - 4 lines] > 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 |
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