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loud backfire on 4.0-HO 5spd


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What would cause a loud backfire on a 4.0-HO when decelerating ~2500rpm. The backfire can be intermittent or a quick string of two or three together. The engine runs good under power, everything seems normal. This is my sons 96 Cherokee with about 200k on the odometer.

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Bad oxygen sensor. If it's slow to respond to changes in engine load (i.e. unburned fuel in the exhaust), when you let off the gas the mix gets rich. The O2 sensor is supposed to see that and shut down the injectors, but if it's slow responding, it allows enough unburned fuel into the exhaust to cause backfiring.

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I have a similar problem sometimes when accelerating i get a few short backfires and then the engine guns it but i don't change the gas pedel at all any ideas? It got really bad and blew a muffler so i changed the distributer, wires, plugs, air filter, serpentine belt, valve cover and exaust manifold gaskets as well as new muffler and tail pipe. Please help someone

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My backfires were related to an exhaust leak. Once I fixed my exhaust/intake manifold gasket i've been good to go, not a single backfire.

Good point.

 

This relates to the oxygen sensor, too. A leak at the exhaust manifold can result in the manifold sucking IN air, which makes the mixture seem lean as the exhaust stream passes the O2 sensor -- so the sensor tells the ECU to send MORE gas through the injectors. Several years ago, a guy in a Cherokee on a NAXJA-NAC trail ride managed to crack his exhaust manifold out on the trail. It was late, and getting dark, so he tried to drive it back to the trail head. Not only was he getting serial backfiring, after a short distance we popped the hood and the exhaust manifold was glowing red.

 

So, yes ... a leaking exhaust manifold can result in backfires.

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Found a hairline crack in the lower portion of the header, see photo. On deceleration this would probably flood the O2 sensor with cold air and trigger the ECU to dump more fuel to the engine. This crack goes all the way around the pipe!! I pulled out the MIG and did a rough patch. My son want to order the Edelbrock header, $372 from AutoZone, are they any good?

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