Rlbcrb1

04` grand Cherokee Laredo 4.0L 2wd started to miss and run rough when Ideling and when trying to accelerate or going up hills but no warning lights come on had the guy at the parts store check the diagnostics and he said that it was missing on all cylinders so I sat in the parking lot for a few minutes checking the ol' interweb and when I started it back up and left it seemed to be running fine.

dave1123

I'm going to pull the old rabbit out of the hat and say "I suspect the coil rail." Those things seem to short out frequently when they get hot after a lot of miles of service and are cheap and easy to replace. One other thing could be carbon buildup from the CCV system, but that usually clears after a hard run.

Rlbcrb1

Thanks! I found new coils for around $60 and I'll put new plugs in as well and hopefully that straightens the ol' girl out. I forgot to mention that it had sat for about 6 months before I bought it

dave1123

Ah, Yes! Moisture is a killer. The problem is with the design of the coil rail. There are 3 coils that fire 6 plugs in pairs on both compression and exhaust strokes. Usually the #1 coil shorts on the link to #6 which fires at the same time, HOWEVER if moisture gets into the epoxy surrounding the coils, anything can happen.

After saying that, I sincerely hope that's your problem. Replacing the coil rail seems to be most mechanic's first attempt at fixing a misfire problem.

My jeep had a problem with #5 misfiring after a long idling period. My buddy the mechanic suggested replacing the rail because he thought I was getting a weak spark in #5. That's what was the problem. It was carboning up because of oil from the CCV and the spark wasn't hot enough to burn it off.

Don't forget to replace the plug boots as well. They generally don't come with the rail.