I just bought a '98 Cherokee 4.0, and I'm regretting it. Engine ran a bit rough at idle when I looked at it, but figured it just needed a little TLC. Well, it has a P0301 (cyl 1 misfire) that I can't figure out. This is my first real experience with a 4.0, so I'm hoping you guys with more experience can help me out.

I have:

swapped plugs w/ cyl 2 (cyl 1 plug did look a little darker, but wasn't wet)
swapped plug wires w/ cyl 6
swapped fuel injectors w/ cyl 2
swapped fuel injector connectors w/ cyl 2
inspected cap and rotor (they look new)
checked compression in cyl 1 (130 psi)

Between each of the "swaps", I cleared the code. Then I started it, and after a few minutes of idling, P0301 would show up as a pending code.

It has 181k miles on it. Any help is much appreciated!

 

I'd change the cap. Can't always see faults that might cause a misfire.
If this is a new-to-you Jeep, may as well change the Cap, Rotor, Wires and Plugs anyway to get started on solid ground. They're periodic maintenance items anyway.

No reason to regret your purchase, this stuff is pretty much SOP for used car purchases. Once you get the Jeep sorted out you can expect 300K miles or more out of it without major drama.

 

Thanks. I do have a semi-update. A co-worker had sent me a link to the public library's auto repair section, where I can read TSBs. There is a TSB about misfires, and the fix is to use Mopar Combustion Chamber Cleaner and replace the valve springs (apparently with a revised part). Hopefully the local dealer has some cleaner on hand, and I'll try it this weekend.

The previous owner had recently replaced the plugs, wires, cap and rotor, apparently trying to solve this issue. I might just replace them again, though. I don't think he used the best quality parts.

Keep the ideas coming...the more things to check, the sooner I'll (hopefully) find it. Thanks!

 

Haha.. TPS, IAC valve, and or o2's..

I laugh because of the empathy for your situation.. I've got a doozy that for now is lying dormant, but I've yet to solve it.. Good luck!

 

Update: I performed the Combustion Chamber Cleaning procedure, then cleared the code. It didn't come back through three different warm up/cool downs, which included about an hour of idling and driving at varying engine speeds. Then it went back to doing what it did before: P0301 as a pending code after a few minutes of running. So, the CCC made a real difference, at least for a while.

I had my daughter crank the engine over while I watched, and all rockers seemed to move approximately the same amount.

I think my next step is to replace the valve springs on cyl 1 per the TSB, and possibly run the CCC again. Any input is welcome. Thanks!

 

Hello Belvedere, I've got a similar situation with a misfire in #5 on my '99 xj. Did you replace the valve springs?

 

Yes, just on cyl 1. It ran smoother after that. So far, after just over a week of driving, no misfire code (knocking on wood!). Best wishes with yours, and let us know how you turn out!

 

So after a lot of amatuer troubleshooting I finally took it to a pro. Turns out fuel injector #5 is out. That was my guess and feels good I was atleast correct, I just didn't have the equipment to check it and confirm.

I declined to have them do it, just paid for diagnostic. I will hopefully get started on it tonight and have a working jeep to take to work tomorrow.

**fingers crossed**