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Unread 05-29-2013, 07:49 AM   #1
cbrant
Registered User
2004 LJ Wrangler 
 
Join Date: Dec 2011
Location: Ames, IA
Posts: 221
Good morning! So, we got this 98 Cherokee Classic 4.0 last summer and it having a misfire issue. It would blink the CEL under load and pulled a cylinder 6 misfire code. Replaced the plugs and wires last fall which fixed the CEL. The 6th plug was extremely gunked up.
Fast forward to this last weekend. It rained a lot, and we started up the Cherokee which began to high idle. Then it started misfiring and immediately threw the cylinder 6 code. Okay, no big deal. Pulled the 6th plug, it was good. Warrantied the wires, so they're new again. Checked the TB and it's clean. Reset ECM and if continues to misfire, but not under load. Just at idle, with occasional backfire. Hasn't pulled another code just yet.
So I'm not sure where to start. Did a bit of research and I think I should test the TPS and CPS? Maybe compression test the cylinder? I thought maybe fuel injector, but am not sure about that. I am thinking about trading the TPS from my 99 TJ into it so see if it changes. So any help or thoughts are appreciated. I'm sure you guys can give me some ideas. Thanks!

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Unread 05-29-2013, 02:41 PM   #2
cbrant
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2004 LJ Wrangler 
 
Join Date: Dec 2011
Location: Ames, IA
Posts: 221
Evening bump.
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Unread 06-12-2013, 03:07 PM   #3
cbrant
Registered User
2004 LJ Wrangler 
 
Join Date: Dec 2011
Location: Ames, IA
Posts: 221
CEL now blinks when accelerating hard again. I realize that you aren't supposed to drive it if this happens. Can anyone help? Is this a heatsoak issue?
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Unread 06-12-2013, 05:57 PM   #4
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,467
A TPS generally doesn't cause a single cylinder misfire. And the same for the crank sensor. This is how I would proceed if it were my XJ.

1. Swap the fuel injector from cylinder 6 into a different cylinder and see if the misfire follows.

2. Run a compression test on ALL cylinders and see where cylinder 6 sits in comparison to the others. The compression spec for the 4.0 is 120-150 with no more than a 30 psi variation between cylinders. Can't ignore the possibility of an internal engine issue. A compression gauge can be rented inexpensively from many parts stores; call around for availability.

Good luck and keep us posted!
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99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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