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Unread 08-08-2012, 02:42 PM   #1
BrianGr
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1998 TJ Wrangler 
 
Join Date: Dec 2011
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4.0L bogs down and backfires

98 Wrangler with the 4.0L and 5spd trans. Jeep runs fine the first time you turn it on, but when it is shut off and restarted while still warm it begins to run like crap. It will bog down, hesitate for a few seconds, then put out a few massive backfires. After it backfires the problem will clear and it will run fine until shut off again.

This will happen everytime the jeep is started while warm. Has been happening for about the last two months. Since the problem arose i changed the plugs, distributor cap, and rotor. Ignition wires seem to be in good condition, as they arent brittle. Have also run two treatments of fuel system cleaner through the tank.

Fuel filter for my year is the non-serviceable style.

EDIT: Replaced Ignition Coil too

Any ideas as to what the issue is here? Im looking for suggestions as to what parts to throw at it next...


Thanks,
Brian

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Unread 08-13-2012, 05:03 PM   #2
BrianGr
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Removed throttle body, cleaned it and reinstalled with new gasket.
Replaced Ignition coil.

No changes. Any thoughts?
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Unread 08-15-2012, 07:10 AM   #3
BrianGr
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bump
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Unread 08-15-2012, 07:25 AM   #4
PathWJ99
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Maybe try the crank position sensor?
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Unread 08-15-2012, 07:51 AM   #5
BrianGr
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I replaced the CPS in december. I suppose its possible i got an inferior replacement, but wouldnt a bad sensor cause a problem all of the time? This only happens when the jeep is restarted while still warm

thanks for the response, and ive been wrong before, so if youve seen a cps cause an issue like this please correct me
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Unread 08-15-2012, 10:10 AM   #6
PathWJ99
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Well mine would drive fine for a while then after it had going for maybe 15-20 minutes it would start acting up then die on me. When yours shuts off will it restart or dose it need to cool down a bit?
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Unread 08-15-2012, 10:31 AM   #7
BrianGr
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The jeep starts right up. The problem is when you get going. It will bog down hard enough to throw you forward in your seat, then backfire. Sometimes one really loud backfire and sometimes it will backfire for fifty yards down the road. After the backfiring clears it runs fine until shut off again.
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Unread 08-15-2012, 10:36 AM   #8
PathWJ99
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That's strange. If I were you I would jump on a big thread that is already started and that is your year and model. You might get more answers that way. But I kinda sounds like something's clogged. I would maybe look at the fuel pump relay next. Start with the little things. I also have the lifetime fuel filter but I decided to put a new one in anyways.
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Unread 08-15-2012, 07:47 PM   #9
BrianGr
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Strange indeed. Im sure its something stupid, thanks for the help
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Unread 09-11-2012, 08:05 PM   #10
BrianGr
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Havent done anything to the jeep, and the problem still persists, so still looking for answers.

Quote:
Originally Posted by PathWJ99 View Post
But I kinda sounds like something's clogged.
Didnt realize it at first, but wouldnt a backfire (the jeep backfires) indicate that its overloading on gas, not starving?

What really kills me is that only does it once, then clears and goes away. Makes it that much more difficult to trouble shoot. I know the fuel pump relay was suggested above, and I need to check it just to be sure, but if the issue was the relay wouldnt the issue be constant?

Probably a longshot, but could it be related to spark advance? Is spark advance on these vaccum operated? I know vaccum leaks cause issues.
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Unread 09-12-2012, 07:49 AM   #11
cruiser54
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Check the fuel pressure before proceeding any further.
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Unread 07-25-2013, 09:12 AM   #12
marmul
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Hi BrianGr, I have the exact same issue. Have you resolved the issue? In addition to the brief backfire/bog i have hesitation on acceleration at about 1800 to 2200 rpm. My MPG is between 13 - 14.
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Unread 07-28-2013, 08:22 AM   #13
Flyin
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I've got a '96 ZJ 4.0 that also does this. Started thinking it was a leaking injector & upgraded them with no change. Also swapped to the new style intake, new crank sensor, TPS, MAP, cleaned the IAC & TB. CAT was gone when I bought it so I put a universal Magna flow cat & new exhaust on it. I'm getting an O2 code (rear sensor) & will replace that soon. Also getting 12-14 mpg.
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