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Unread 10-16-2007, 08:33 PM   #1
Flammable
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1998 XJ Cherokee 
 
Join Date: Jan 2006
Location: Chadron,NE
Posts: 186
Help 4.0L Guros!

Alright, this has been an ongoing battle with my jeep. I'll start from the beginning.

One day, all the sudden my jeep started to backfire and load up with fuel. Well we thought it was the cat, so we cut it out, to be replaced later of course.

A few months go by, with no issues, and now the backfire is back. After I put my lift kit on my jeep the O2 sensor harness fell out of its bracket and wrapped around the driveshaft. So some electrical tape and a new sensor, later, it should have been working right? Nope, code now says that the front o2 sensor is out. So we replace that one.

Fast forward to tonight, the backfiring is now even worse, so I decide to take the tube off the throttle body. What I saw inside was a whole bunch of carbon, fuel, and just gunk.

Now, I would go out and check the timing right now with no problem. Except for the fact that the FSM says not to adjust it. Is there a way I can safely play with the timing? As far as I know there is a base setting, and the computer controls the rest.

I forgot to mention that it only backfires while accelerating, and sometimes higher in the RPM's.

Gas mileage has also gone down, way down. So heres my theory. Because of the backfire, its blowing all of the fuel and air back out. So then because of the o2 sensors, the computer says alright, not enough fuel, so it pushes even more fuel in until finally there is spark. Plausible?

Sorry this post is kinda jumpy and long, I'm just getting really frustrated with my jeep at the moment. Thanks in advance.

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1998 Jeep Cherokee 164k 4.5" lift RE lift, 33x10.50x15 Maxxis Bighorn, 16x8 wheels (ouch), 4.56, APN Header
Jeep Owners Named Nick Club #2
Nebraska Jeep Club Member #2
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Unread 10-16-2007, 09:29 PM   #2
ajmorell
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Join Date: Jan 2006
Location: Santa Fe, New Mexico
Posts: 7,144
You said you saw a lot of carbon build up in the intake? Try running a bottle of Mopar Combustion Chamber Cleaner through it and see if that helps clean it out any.
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Unread 10-16-2007, 09:42 PM   #3
Flammable
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1998 XJ Cherokee 
 
Join Date: Jan 2006
Location: Chadron,NE
Posts: 186
Yeah, that might clean it out, but it's only going to return because of the backfiring correct?

I figured I might just leave it dirty (since everything thats in there is combustable anyways), just so that the dealership can take a look.
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1998 Jeep Cherokee 164k 4.5" lift RE lift, 33x10.50x15 Maxxis Bighorn, 16x8 wheels (ouch), 4.56, APN Header
Jeep Owners Named Nick Club #2
Nebraska Jeep Club Member #2
NightCrewMember #104
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Unread 10-17-2007, 08:30 AM   #4
75SV1
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Join Date: Sep 2003
Posts: 1,062
Its a long shot, but check your timing. Rotate the engine for '0 degree' Top Dead Center. Since your is a 98, take the cap off the distributor. Check that the rotor is pointed at the #1 position. You might mark it on the outside of the distributor with a marker. I am thinking that you might have a streached timing chain. At 88k, its possible. That might be throwing off your timing. Still, there might be other problems.
Tom
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Unread 10-17-2007, 11:03 PM   #5
Flammable
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1998 XJ Cherokee 
 
Join Date: Jan 2006
Location: Chadron,NE
Posts: 186
Well up at the shop today we decided to run some fuel injector cleaner through it. Didn't help.

I would suppose its the timing chain, camshaft, or whatever else, but wouldn't that mean that it would be backfiring all the time? Or can the computer really confiscate for major issues like that?
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1998 Jeep Cherokee 164k 4.5" lift RE lift, 33x10.50x15 Maxxis Bighorn, 16x8 wheels (ouch), 4.56, APN Header
Jeep Owners Named Nick Club #2
Nebraska Jeep Club Member #2
NightCrewMember #104
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Unread 10-22-2007, 06:17 PM   #6
Nonya
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1998 TJ Wrangler 
 
Join Date: Feb 2007
Location: Nice
Posts: 1,653
Bad camshaft,pull the valve cover and check valve lift.If you find any that are not moving the proper amount you will probably need a new camshaft and lifters.The reason it backfires under a load is an exhast lobe is worn off and is not opening far enough to let the hot gases out.When the intake valve on that cylinder opens the hot gases blow back into the intake manifold causing the backfire.Symtons would be idles ok but start giving it throttle and it starts backfiring faster and faster back through the intake.Good Luck.

Last edited by Nonya; 10-22-2007 at 06:43 PM..
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