I have an 01 XJ Sport with the 4.0. I have replaced the head with a new one with the heavier casting. Before the head replacement it ran fine. Now on cold startup it runs great, no issues but after about 10 minutes of driving it develops a bad miss and throws P0304. Since then I have installed new Champion plugs, new coil rail and rebuilt Bosch injectors. I clear the code, cold startup, runs great until warmed up again then code and miss are back. It only misses and throws the code while warm, normal operating temperature. I have read a lot about the heat soak for #3 but can it also cause that in #4? Or am I missing something else? Oil and coolant look good. I love the Jeep, no rust, 210K miles, 4.5" Rough Country lift on 31" M/Ts, roof rack with KCs, but I am losing patience. I have searched the forums and found a lot of great answers but nothing too specific, maybe someone has had the same thing happen.

 

Doesn't sound like heat soak (in the traditional 00-01 XJ sense we're all used to) if it's happening while running. What coil pack did you use? Mopar?

Try swapping the injectors from cyl 3/4 and see if it repeats or the code moves with it. If no difference, do the same for plugs. If still no difference, i would think it to be thermal failure of a sensor or new component (like the coil pack). Not sure if the new head being out of tolerance could cause this, so I won't opine on that...

 

Mopar coil rail. Old one tested fine though. Moved injectors and plugs, no difference. Thanks for the reply.

 

Could be a intake valve seat issue. Have you checked compression?

 

Compression is good across the board. Tracking this down has me going insane!