2000 XJ 4.0 misfire problems

[Redsnake]

Some time back before I replaced the OE Coil Pack w/the Coil/Plug Wires combo... I had an intermittent miss on my '01 XJ. I couldn't figure it out so on a whim... changed the OE Coil Pack w/a new unit... Cleared all my miss and idle trouble right up and my XJ ran great. No outside visual evidence that the OE coil pack was bad... other than maybe it having 120K miles...

Still, just throwing a new Coil Pack on it to 'test' it is not cheap. Perhaps if you know anyone nearby that has an '00-'01 w/a good Coil Pack would allow you to swap out and test it. Other than that I don't know ... Good luck

 

[2kXJ4wd]

Well seeing as how this one has 178k on the clock I figured a lot would need to be replaced. I have a budget of 5k for the entire build to include getting it running good. Every dollar I spend on getting it to drive right is a dollar less on lift, tires and stuff. That's why I am not trying to throw parts at it. I got it for a steal but I am soon going to go over budget. I will try a new coil rail and cross my fingers...again.

 

[2kXJ4wd]

Replaced coil rail with a brand new one, still misses and idles like crap.

 

[Redsnake]

That sucks... nothing like 'throwing' parts at ones rig and not solving the problem (been there done that WAY too many times...)

Crank Position Sensor, Coil Pack, Throttle Position Sensor, Fuel Injectors, Fuel Pump, Bad/Week Battery, Bad Ground, O2 Sensor(s), Clogged Cat, Bad Vacuum (cracked line somewhere), Cracked 0331 Cylinder Head... that's about all I can think of that would cause the poor idling running.

 

[md21722]

On my 01 XJ I was getting misfires at idle about 6 years ago. Like you I changed the crank & cam sensors, coil pack, and a fuel injector. It turned out to be a mechanical problem with the exhaust valves. Compression test showed low compression on all cylinders that were throwing codes. Head came off, valve seats machined, and the problem went away.. permanently.

Years later I found a TSB that outlined a procedure to check the exhaust valves were rotating. Remove the rockers and check the valves. If there is a line rather than a bullseye, the valves aren't rotating. The TSB outlined a procedure to manually rotate the valves and said the problem could clear if there isn't excessive carbon buildup that would prevent it. I'd try that plus some Mopar Top End Cleaner and if it doesn't fix it, pull the head and take it to the machine shop.

 

[randomguy310]

I did the same thingbut turned out to be my cps.. 1st bought on from ebay.. and it was junk then bought one from autozone, seems to be fine

On my 01 XJ I was getting misfires at idle about 6 years ago. Like you I changed the crank & cam sensors, coil pack, and a fuel injector. It turned out to be a mechanical problem with the exhaust valves. Compression test showed low compression on all cylinders that were throwing codes. Head came off, valve seats machined, and the problem went away.. permanently.

Years later I found a TSB that outlined a procedure to check the exhaust valves were rotating. Remove the rockers and check the valves. If there is a line rather than a bullseye, the valves aren't rotating. The TSB outlined a procedure to manually rotate the valves and said the problem could clear if there isn't excessive carbon buildup that would prevent it. I'd try that plus some Mopar Top End Cleaner and if it doesn't fix it, pull the head and take it to the machine shop.

 

[2kXJ4wd]

Okay so I have an update. Checked the IAC and it is clean as a whistle and the plunger moves in and out when I turn the key. Checked fuel pressure just now and it is a solid 50 psi and it holds there for about 10 mins and slooooowly drops after key is off. I guess next would be to pull the head. I have a new CSF radiator and new water pump, t-stat, and hoses. I suppose I should pull the head while I'm that deep in it. Anything special I should look for other than the bullseye and the infamous crack on the 0331 head? I have a coolant leak but it is from the original radiator with 180k on it.

 

[2kXJ4wd]

I did the same thingbut turned out to be my cps.. 1st bought on from ebay.. and it was junk then bought one from autozone, seems to be fine

I really never thought it was the crank sensor. I have 0 problems with it starting, hot or cold. My guages never went crazy or stopped working, and my scan tool connected no problem after i fixed the ground for the port. I had a feeling it needs a head job from the start. It is a money pit now and there is only 1 way to get out, keep digging lol.

 

[2kXJ4wd]

Pulled the valve cover and it looked like new. No sludge or traces of coolant anywhere. Pulled rockers and pushrods and they all look great. Good wear patterns on the valve stems. Pulled the valve springs and they all look good too. Gonna drive it until it dies, while building a stroker to swap when it does.