I've got a '98 XJ Sport with 258,000 miles on the clock. It's never idled perfectly but it's always run pretty well otherwise.
For a while I've suspected that it may need a valve job since it idles a little rough, but anything above idle speed is silky smooth. Just a month ago it passed emission testing with flying colors - so clean that it actually got a "fast pass", so I figure it can't be too bad. No check engine light or other bad behavior other than this rough idle.
It's an extra vehicle so it doesn't get driven all the time, but I've been driving it more lately so I can narrow down what needs to be done before winter. Tonight I decided to do a compression test. All six plugs looked good and gaps were right on 0.035". Cold compression numbers with the fuel pump relay pulled were as follows:
Cylinder 1: 120psi
Cylinder 2: 150psi
Cylinder 3: 125psi
Cylinder 4: 138psi
Cylinder 5: 160psi
Cylinder 6: 158psi
Thinking that maybe my case for a valve job is looking more legitimate now, I dribbled about 0.5oz of Marvel Mystery Oil into cylinder 1 and tested compression again in just that cylinder. It came up to 125psi. Not enough to indicate rings in my mind.
Now after putting everything back together I have a pretty nasty misfire. I started it up and let it idle for a couple minutes and everything seemed normal, but as soon as I drove out of the parking lot to my test loop I could feel at least one cylinder dropping out pretty consistently. I figured maybe it was cylinder 1 since it got a little dose of the Mystery Oil, but even after a couple of minutes of driving the plug didn't seem to clean out. At light throttle it was pretty smooth but with a bit more it was a solid miss.
Pulled back into the shop and grabbed the IR thermometer only to find that all six cylinders registered nearly identical temperatures at the exhaust manifold. At this point I'm thinking I must have done something silly so I double checked the ignition (they were labeled prior to me removing them) and made sure all of the wire were firmly seated on the cap and their plugs.
Found nothing wrong... No check engine light either, just a nasty misfire. What the heck? Finally ran out of time so the Jeep got parked and I drove something else home.
Crank position sensor was replaced in July with a genuine Mopar part. TPS was replaced a few years ago and didn't seem to help with the rough idle. ICV has been cleaned a couple of times - engine idles around 750rpm when it's warmed up. I think the coolant temp sender and MAP sensor were replaced about three years ago too with no change to the rough idle. All grounds were checked and cleaned in July.
When my girlfriend used to drive it every day there was an intermittent CEL that always came back as a random multiple cylinder misfire, but as soon as she stopped driving it and I started, it went away. I attribute this to my more aggressive driving style. She never spun the engine much over 2500rpm. It's been great for the last year until today.
That said, I do believe that at the very least this engine would benefit from a valve job. Maybe I'll find a couple of burnt valves when this head comes off. But what I'm stumped on is a hard miss that doesn't trip the check engine light. Any ideas? Could the Mystery Oil have washed the rings down in cylinder 1 enough to cause a near-total loss of compression?
For a while I've suspected that it may need a valve job since it idles a little rough, but anything above idle speed is silky smooth. Just a month ago it passed emission testing with flying colors - so clean that it actually got a "fast pass", so I figure it can't be too bad. No check engine light or other bad behavior other than this rough idle.
It's an extra vehicle so it doesn't get driven all the time, but I've been driving it more lately so I can narrow down what needs to be done before winter. Tonight I decided to do a compression test. All six plugs looked good and gaps were right on 0.035". Cold compression numbers with the fuel pump relay pulled were as follows:
Cylinder 1: 120psi
Cylinder 2: 150psi
Cylinder 3: 125psi
Cylinder 4: 138psi
Cylinder 5: 160psi
Cylinder 6: 158psi
Thinking that maybe my case for a valve job is looking more legitimate now, I dribbled about 0.5oz of Marvel Mystery Oil into cylinder 1 and tested compression again in just that cylinder. It came up to 125psi. Not enough to indicate rings in my mind.
Now after putting everything back together I have a pretty nasty misfire. I started it up and let it idle for a couple minutes and everything seemed normal, but as soon as I drove out of the parking lot to my test loop I could feel at least one cylinder dropping out pretty consistently. I figured maybe it was cylinder 1 since it got a little dose of the Mystery Oil, but even after a couple of minutes of driving the plug didn't seem to clean out. At light throttle it was pretty smooth but with a bit more it was a solid miss.
Pulled back into the shop and grabbed the IR thermometer only to find that all six cylinders registered nearly identical temperatures at the exhaust manifold. At this point I'm thinking I must have done something silly so I double checked the ignition (they were labeled prior to me removing them) and made sure all of the wire were firmly seated on the cap and their plugs.
Found nothing wrong... No check engine light either, just a nasty misfire. What the heck? Finally ran out of time so the Jeep got parked and I drove something else home.
Crank position sensor was replaced in July with a genuine Mopar part. TPS was replaced a few years ago and didn't seem to help with the rough idle. ICV has been cleaned a couple of times - engine idles around 750rpm when it's warmed up. I think the coolant temp sender and MAP sensor were replaced about three years ago too with no change to the rough idle. All grounds were checked and cleaned in July.
When my girlfriend used to drive it every day there was an intermittent CEL that always came back as a random multiple cylinder misfire, but as soon as she stopped driving it and I started, it went away. I attribute this to my more aggressive driving style. She never spun the engine much over 2500rpm. It's been great for the last year until today.
That said, I do believe that at the very least this engine would benefit from a valve job. Maybe I'll find a couple of burnt valves when this head comes off. But what I'm stumped on is a hard miss that doesn't trip the check engine light. Any ideas? Could the Mystery Oil have washed the rings down in cylinder 1 enough to cause a near-total loss of compression?
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