dch

My 2001 4.0 XJ is having this annoying misfire at any RPMs (most noticeable at idle, of course), but only after it's been running for like 2 or 3 minutes. When started from dead cold, it runs perfectly, then starts misfiring, very regularly, which seems to get worse as it heats up, to the point where it will almost stall when fully up to operating temp. I just replaced the head with a new one, due to the 3/4 cylinder crack issue, but I think it was doing this before that. (It's actually what made me finally replace the head, as it started running so poorly). I have swapped the coil rail pack with a known good one (no change), and swapped the whole throttle body (w/sensors) from another good-running vehicle (no change). No codes are being thrown. Compression is good on all cylinders. New plugs, and they are perfectly clean. Fuel pressure is at 47 psi constant. I checked the rockers and pushrods, they all seem fine. Any help with what to check next would be greatly appreciated!

Roler

Do you have access to a scanner which displays live data?
Sounds like you have a heat related issue
(O2 sensors can do that for instance )

Does it stall out idling or while driving as well?
Did you ensure your fuel pressure stays correct, so once warmed up too?

dch

Thanks for the reply. Live data shows the O2 sensors (all 4) giving what seems like normal readings. It does miss/stumble/hestitate while driving, esp. when pressing on the accelerator again after coasting, but also under steady acceleration. Fuel pressure stays the same when warmed up.
It's strange, occasionally if I just let it idle, it will pop back into a few seconds of "normal", perfect idle, then start sputtering/missing again. And the idle is a lot worse after driving a while.
Here's something interesting: I have a brand new spare temp sensor, so I took the connector off the one on the t-stat housing and plugged it into the new one, and just let it hang there with the fan air blowing on it. Jeep ran perfectly normal for a lot longer. But once warmed up, it runs better, but still has the missing problem.
It also started throwing P0300 (multiple cyl misfire), P0305 (cyl 5 misfire), and P0302 (cyl 2 misfire) codes after running a while.

exasemech

1)if you don't have a scanner that shows live data, buy one. They don't cost much and if you have a 25 year old OBDII vehicle you need one. Going to the car parts store to get it scanned is silly.
2) with the scanner you have or will be getting look at the short and long term fuel trim. Given what you've checked already it's likely either running lean or is an electrical connection that gets funky when it gets hot. Do the grounds.

dch

Well, I decided to pull the valve cover and watch the rockers and valves with the engine running. It looked a lot like #5 exhaust and #2 exhaust valves weren't going down as much as the others. Thinking about cam wear, I measured the deflection of each, from the head to the top of the valve, turning the engine by hand. Sure enough, #5 exhaust measured 1.790 in, and #2 1.710 in, while the other exhaust valves were in the 1.57-1.62 range (meaning these were being depressed further). That would explain the P0302 and P0305 codes. So apparently either I have collapsed lifters, or the cam is shot? There is a ticking noise that's pretty noticeable. None of the rockers are loose at all, though.