Rough Idle, Dirty Coolant

I have a '98 XJ 4.0 with 186XXX miles on it. Bought it knowing there was low compression in cylinder 5 but not knowing what was causing it. Did a semi-long trip (260 miles) and at the end the engine was idling low and rough, and the coolant was dirty (coolant may have been dirty before). No over heating problems, oil pressure looks fine, but my coolant and oil both seem to be slowing leaking and the check engine light is on (but i don't know what code its throwing). I checked the oil for cloudiness and foam and found none. i am at a loss for what is going on. My initial assumption was headgasket but i talked to a mechanic and he thought it could have something to do with the valves. any thoughts, ideas?

Download the Check Engine Light codes and fix the known problems. Perform the deferred routine maintenance ie, tune-up and change fluids. Determine the locations and amounts of fluid leakage, decide it the leakage is a fatal problem, or not. Fix the leakages. Test the compression and determine if it is acceptable or not.

Great tips above

All I could add is have the systems pressure tested after you've changed the fluids. The cooling system can be done and I think the valves can be done too.

Head gasket would be much nicer than valves...

1. Run another complete compression test on all cylinders. First dry, then wet (add a teaspoon of oil to the cylinder). Gotta get good data to figure out what is going on inside that engine. The spec for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders.

2. Always download codes when a check engine light exists. Anything else is guesswork. Use the OBDII technology provided to you by your Jeep. For a 98, you'll need a code reader, a scan tool, or some stores like Autozone will download them for you for no charge.

Quick clarification while on the topic...

Is that no more than a 30 psi variation within 120-150 or only when the numbers fall below the spec? Say what if you have 120 in one cylinder, 150 in all the rest? That's a 30 psi difference, but within spec.

walkerc said:

Quick clarification while on the topic...

Is that no more than a 30 psi variation within 120-150 or only when the numbers fall below the spec? Say what if you have 120 in one cylinder, 150 in all the rest? That's a 30 psi difference, but within spec.

Click to expand...

150 in 5 cylinders and 120 in one cylinder would technically be in spec, but would also tell you that the cylinder with 120 is not working as optimally as the others and that something is likely starting to occur there.