New XJ owner...1996 with 150k. After a couple weeks of driving it, it started running VERY rough at idle when cold for a couple of minutes and then would get better. Still was never what I'd call smooth. This coincided with a coolant and thermostat change, which was done because of overheating shortly after startup. I don't think it's relevant but didn't want to leave out the detail.

Pulled plugs this morning and I think what I'm seeing is oil fouling in #4 and possibly coolant burning in #5. See attached pic...

Thoughts?

 

Looks to me like #4 isn't firing.

 

Agree with Cruiser54. Space on #4 looks very tight.
Clean, open up the gap, try again.

 

Good eye!!! I'm still on my first cup of coffee.

 

I'd try a new plug in #4, gapped to .035

As long as you're at it, would be a good time to get a compression test done. It's a great "snapshot in time" of the condition of the engine. A compression test can be very helpful in diagnosing anything internal that might be going on. Great data point for determining direction. Do all 6 cylinders, not just #4.

The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

 

How many miles on these plugs?
#4 looks like it's oil fouled.
#3 looks like has splash deposits
#5 looks leaner than all the others.

If it were mine, I would install six new, properly gapped plugs, drive it about 50 miles and pull them out. Take the same picture and compare them to each other and this photo.

KenR

 

Check the cap and rotor too. They may be worn. My 99s cap/rotor were completly shot when I purchased the Jeep. Quick replacement of everything but the wires and my MPG increased. Smooth as buttah too now.

 

Judging by how clean the bottom of the threads are on #5, It looks like it could be a replaced plug newer than the rest and correctly burning lean as these do, or it could "possibly" be burning coolant in that cylinder if it is the same age as the rest.

 

I installed new plugs after taking these out. I like the suggestions though...I'll drive a bit, pull and compare, and do a compression test while they're out.
I pressure tested the cooling system yesterday and it held for the most part, but had dropped to 15 psi after 15 minutes. If it is leaking coolant into a cylinder it's just starting.

I'm sure I'll know more after the compression test - thanks all for the suggestions.

 

How is it running?

 

About the same. A little less rough at cold start (not lumpy like it was, but not smooth), smooths out when driving, gets a little bumpy when stopped.
The thing I'm going to need to figure out is whether the oil in #4 is coming from a valve seal or if I've got bad rings. Compression test should reveal a lot when I get to it.

 

Something to take note of when you pull 4 out again is the gap. It looks to me like it could have physically been impacted by something. Carbon build up on the piston maybe. Because I have never seen one up against the electrode like that even right out of the box ungapped. Looks like something hit that one.