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Old 02-25-2011, 12:21 PM   #1
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Default backfire through exaust???

I found my motor running like crap is due to backfiring through the exaust. What should i be looking for to fix this?! when i rev it up it "POPS" and i cut exaust at the tranny "thinking catalac conv was clogged" and can really hear the popping now....please help!

History: ran fine then fuel pump quit, replaced fuel pump, filter plugs wires and cleaned that brass cone sensor in the throttle body.
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Old 02-25-2011, 02:14 PM   #2
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bump "would a cps cause this?"
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Old 02-25-2011, 06:14 PM   #3
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ok another update....i pulled rubber boot off at the manifold that runs to the vavle cover"the small line" and it seems to smooth out until i put the line back i think the plastic line is restricting the suction but i dont know why it would cause it to run that poorly. its still sucking just only about half of what its sucking straight from the manifold like the line has a parcial block or something....
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Old 02-25-2011, 06:24 PM   #4
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your backfiring is caused by a lack of back pressure. meaning your exhaust is to free flowing.
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Old 02-25-2011, 08:13 PM   #5
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I thought a backfire was "backwards" thru the intake....no?
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Old 02-25-2011, 10:07 PM   #6
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no it was shooting fire out the exaust so i think there is a small problem somewhere and i think it has something to do with the pcv system is there something in that messed up system that gets cloged and needs replaced or something?
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Old 02-26-2011, 12:34 AM   #7
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Quote:
Originally Posted by djb383 View Post
I thought a backfire was "backwards" thru the intake....no?
no need to be a smartass its a real thing. with 2k post i'm surprised you've never heard of such a thing.i think this is the culprit, especially if his problem started after he cut the exhaust. i'm not going to take the time to explain how the a backfire from lack of exhaust works but the OP said it back fires when he lets off the gas also indicating lack of back pressure.

your problem could also be from to much air getting sucked in (new intake maybe?), bad timing (which is rare) or its running to rich when you get on the gas.

i think your problem is from the exhaust though.
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Old 02-26-2011, 06:41 AM   #8
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Backfiring through the exhaust is often caused by a "rich" fuel/air ratio. Test fuel pressure with a GAUGE, (fuel pressure regulator still in play here) map sensor, coolant temp sensor for starters.

You want to see approx 31 psi at idle for your vintage. Then disconnect the vacuum line to the fuel pressure regulator and pressure should jump to approx 39 psi. And be sure that the vacuum line to the FPR is pulling good vacuum and is not wet with fuel. If it has fuel in it, the FPR is ruptured.

And sounds like you have some CCV blockage on the lines on the valve cover. Not uncommon at all on the 4.0 engine. Best fix is to remove the valve cover, throughly clean the cover, replace all the CCV hardware (about $40 from the dealer or Napa to name 2 sources), a new valve cover and you're good to go.
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Old 02-26-2011, 07:22 AM   #9
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I'm thinking that probably this is due to gas not being burned properly and accumulating in the catalitic converter where it eventually ignites. When is the last time you replaced the spark plugs, wires, cap and distributor? O2 sensor may cause this. My exhaust is straight on mine and it has 320 000km no backfire.

troubleshooting, always start with the easiest fix.

backfire = fuel igniting in the exhaust = fuel not being burned in the combustion chamber....
if you want to save time, go to a garage and have it scanned for error codes.
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Last edited by 97CherokeeDave; 02-26-2011 at 07:28 AM. Reason: update
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Old 02-26-2011, 09:10 AM   #10
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Quote:
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no need to be a smartass its a real thing.......
Wasn't tryin' to be a smartass......a backfire can occur either ahead of the combustion chamber (intake) or after (exhaust)....look it up.

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Old 02-26-2011, 02:53 PM   #11
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on fuel injection its pretty rare to see a backfire in the intake anyways....I've only seen it on carbed cars.
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Old 02-26-2011, 05:37 PM   #12
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Backfires can occur either in the intake or the exhaust on any engine.
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