Mel

Help, please: 2001 Cherokee Sport, 4.0, auto, air. Had a coolant leak for a long time which finally resulted in overheating. Around the time of the overheating incident the engine began serious misfiring. Pulled the head, which was cracked, and replaced it. At the same time replaced the plugs and tested compression at 180-210 on all cylinders. The vehicle runs smoothly for a couple of minutes until it approaches operating temperature. Then it begins running very roughly and sets codes indicating multiple misfires in cylinders 4 and 6, which were the same cylinders misfiring before the valve job. The misfires are much worse between 1500-2200 rpm, so keeping the rpm's high helps run a little smoother. Also, after warm-up, idle speed drops low enough to cut off. Any thoughts?

Mel

Really need some ideas where to go next. Thanks all.

Copperhead

Not positive..but I think I remember reading something here about a bad temperature sensor causing this type of problem....Try doing a search for "temperature sensor" under the search function.....

Mel

We removed and cleaned the injectors on cylinders 4 and 6, then swapped them with cylinders 1 & 2. After driving for a few miles, then checking the codes, we got the same as before: multiple misfires, misfires in cylinders 4 & 6. So, obviously the problem is not in the injectors. Could be the injector harness. How do you test that?
Is there any way to test the coil pack?

Mel

By the way, the temperature gauge seems to be working fine. So, that should mean that the temperature sensor is okay, right?

nvminer

Not sure if this is what is happening in your case but here goes. A while back I asked the same thing on a different forum (I am having the same misfire problem) and I was told that there is a TSB for it. Basically when the 4.0L I6 engines were designed some of the injectors (4/5/6) were placed to close to the exhaust manifold (or engine block, can't remember and I don;t have my jeep to go look). This would cause the heat coming off of the manifold to vaporize the gas inside of the injector thus causing a misfire and/or rough idle. I was told that "heatsoak" was causing the problem (basically letting the car sit after it had been operating at full temp, allowing the heat to really get to the stagnant gas in the injectors). I have this problem both while driving and after parking and restarting. The solution from Jeep was this stupid little paper heat shield that was supposed to go around the injectors. Needless to say it only worked for a few days until they shields rattled themselves loose. I have heard guys say they wrapped all of the injectors in heat resistant tape and it made a difference. Hopefully this helps your problem!

nvminer

Pretty sure this stuff would work:
http://www.summitracing.com/parts/THE-14000/?rtype=10

I'm going to give it a try and I'll let you know

Copperhead

No..I don't think so...I believe the temperature sensor sends info to the cpu which uses it to adust the air/fuel mixture...I think there is a separate sending unit that the gauge uses...Someone can correct me if I'm wrong...

ca441970

I wrapped my injectors and my entire fuel rail with a roll of $16 heat tape I got from Advance Auto Parts. Yes it did help. I still get the same misfire in cyl 1 but I did notice my idle had improved.

Mel

According to Alldata the engine coolant temperature sensor supplies info for the gauge as well as engine control.

The tech bulletin on heat soak affecting injectors doesn't seem to apply to my situation. For one thing, the missfiring begins as the engine gets close to operating temperature. It doesn't get any worse after long drives on hot days. Our temperatures have been in the upper 90's on the days we've test driven the Cherokee.

How can the coil pack be tested?

Thanks for the suggestions.
Mel

firstime00XJ

I would call Auto Zone or some other auto parts store and see if they can test the coil packs. I'm not sure how one would test the coil packs at home.

djb383

The O2 sensor(s) provide the info to the ECU to adjust the a/f ratio.

Copperhead

Agreed....But I was under the impression that the temperature sensor controls these things until the engine reaches normal operating temperature?..and if so..couldn't it cause fuel/air mixture problems if it wasn't operating correctly?...I'm only going by stuff I've read here...I've got a lot to learn..

huntingman2706217

coil packs stop working once they get warmed up pretty ofter... they are all the same... try swapping them around also and see if the codes follow... also replace plugs if there not already new... you had the 0331 head on your 01 and thats why it cracked... lots of threads started for that problem... jeep got alot of people on that one... at least you caught it before the bottom end went out... my buddy wasnt so lucky with his 00 GC... but i was cause i bought it and stuck a motor in it and made $3200 off the deal... lol

Mel

Still haven't solved the misfire problem.
Put the jeep in a shop for analysis. The mechanic substituted a new coil pack - no change. So, it's not the coil.
He believes it is lifter problem.
The engine runs smoothly for a short while until it warms up. Then the misfiring begins. It has moved on to set a code on cyl 5 now. So, it's multiple misfires on cylinders 4,5,6.
Does it make sense that lifters would be fine cold but begin causing problems on warmup?