I just did a tune up on the XJ (1998, 155k, 4.0, auto). I replaced the plugs (champion copper), wires (Bosch) distributor cap and rotor, and then ran some sea foam through the brake booster vacuum port. All wires are run correctly to the right distibutor post and cylinder. After the sea foam soaked for a few minutes, I cranked it up and it barely smoked at all.

There is a slight, random misfire and a small knock I can here in the engine (even more noticeable when I am under the XJ). I am pretty sure the hesitation at idle/misfire was there prior to the tune-up but it is much more pronounced now. It barely affects the way it drives, but under a load I can tell it is there. The exhaust sucks in when it hesitates/misfires. I put a piece of paper on the tailpipe and it sucks it in and blows it off very quickly each time.

Each spark plug is getting spark from the wire and I pulled each injector plug to see if one made no difference and each injector reacted the same. So a compression test was my next step.

Results:
#1-130
#2-120
#3-105
#4-115
#5-125
#6-130

This was done with an Autozone rental and was my first time doing a compression test. I am pretty sure they are all on the low end. This was a dry test I guess, since I don't know how to do a wet test. I also didn't have the throttle opened all the way either and as I progressed the engine temp dropped below operating temp.

Any help? Am i headed in the right directions? Any other ideas like any sensors I can test?

Thanks guys!

 

Add a cap full of oil to each cyl for wet test.

Throttle iirc should be held wide open.

That one cyl that is really outta range concerns me. They should.be only a few psi different either way...

 

Ok, I am going to try to re-do the dry test with the throttle open. Then run the wet test. The operating temperature thing is a pain since now I have to reinstall everything to get it back up to op temp. I post what I find.

 

Could be a burnt exhaust valve at #3.

 

Could it have been an undergapped sparkplug? While I was putting them back in to run it to operating temp I was re-checking the gap and noticed one was way too narrow. No signs of piston impact. The only thing is I am sure that I checked each one when I put them in. I corrected the gap, installed all the the plugs and wires and now it is idling smooth as silk. I am scared to take the plugs back out to finish the compression test.

 

Weel I decided to go ahead and re-check the compression. This time ensuring the throtle was wide open. Dry test Results:

#1-140
#2-140
#3-120
#4-125
#5-140
#6-140

So My #3 and #4 are definitely loosing compression. But since the misfire is gone I decided to not do the wet test right now.

 

You're done, you solved the problem. Good job with the diagnostics.

 

Yah, so the problem is fixed but I am still curious if there is a reason the #3 and #4 cylinder are lower and out of tolerance for the compression. I have read something about a heat shield but I am not sure if that is applicable to my XJ. Any thoughts?

But yeah the problem is fixed. The diagnostic thing is something I am working on as I am better at figuring out how to fix something I know is broken than diagnosing the problem.

Thanks for the help.