Okay guys, I've got a real headbanger for you and I need some opinions from the mechanical geniuses out there.

Engine is a 2000 WJ 4.0 with 208K miles on it, runs strong, smooth power, new head, new valve seals, no blowby from the crankcase with the CCV removed. Compression is around 180 psi across the board. HOWEVER, it smokes at idle ONLY. Not under power, not descending hills in second gear, just at idle. If you let it idle for longer than an hour, cylinder #5 will carbon bridge it's spark plug and stop producing power of any kind. The smoke also stops. There are no weird noises from the engine, no knocks or piston slap, but depending on how long you idle it, it uses quite a bit of oil.

Because it only smokes at slow piston speeds and low flame temperature, I'm thinking it's a bad oil ring, either broken or stuck. What do you make of it? I'm going to take out that plug, pour in some Seafoam, and let it sit for a while to see if I can get any carbon out of it or free up those rings. Do you think that will do any good? All the other cylinders are running clean, just #5 is a problem.

I guess when waiting for my newspapers, I'm going to have to shut it off, wait until the cabin gets cold, then warm it up, repeating the cycle. Last night I did my entire route on 5 cylinders! That's not good for any engine!

It could also be a bad injector or a bad coil pack on that cylinder. Since you have a new head I'm assuming the valve guide and seal are OK, and the head gasket is also. If a bad injector or weak coil spark cause that cylinder to not fire at idle it will cause oil to be sucked past the rings as the valves are closed when the piston descends on its power stroke, creating a strong vacuum in that cylinder. with that many miles on the engine this effect will be far more than on a low mileage engine. You might try swapping holes with the coil and injector to see if the problem moves to another cylinder. I would get a new spark plug as well, if a plug has been cleaned more than once it usually will not function nearly as well as the other plugs, and if it was cleaned with a wire brush this is doubly true. Years of working on Harleys drove this point home pretty good, I used to have a little sand blasting machine that you screwed the old plugs into to clean them, worked great too. Wire brushes coat the internal porcelain insulator with metal shorting them out, and solvents often do the same thing,

With your problem I would switch coil packs with #6 cylinder, injector with #4 cylinder, and install a new plug in #5 cylinder. If the problem moves to either of the other cylinders you will know the part you moved to that cylinder is bad. If problem remains in # 5 cylinder then you know you have a ring problem. Hope this helps, good luck and God bless.











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Thanks for your response, but I failed to mention all the injectors are new Bosch 4-hole and I never reuse plugs. They are all new NGK ZFR5Ns. The one that's getting fouled gets replaced with a good one from the last set that's got less than 200 miles on them. The 4.0 coil pack is a self contained unit and you can't swap coils on it. I know #5 is firing because there is no pulsation in the exhaust at idle and the engine pulls well at acceleration. #2 is firing cleanly, so #5 must be as well. The coil pack engines fire on the exhaust stoke as well. 3 coils, 6 cylinders. Usually when there's a problem with the coil pack, it appears with #1 and #6 because there is a long conductor to #6 coming from the first coil and it shorts to the case inside.

I've had engines with cracked rings, cracked pistons, or just badly worn out rings, and nothing acts like this. Compression in that cylinder is 180 psi and comes up quickly on the first stroke when testing. It will behave as long as I don't let it idle for extended periods and only smokes at idle. If you rev it up, the smoke clears immediately but returns as soon as it settles down to idle. I'm a pretty good diagnostician, especially when it comes to mechanical internals, but this is driving me bonkers!

Something just hit me. I've never owned an engine that used the waste spark process before and I'm wondering if this is what's causing the fouling, the fact that it's firing with no fuel heat to clean it out.

With all that said I would have to agree that you likely have an oil ring problem. I don't see that putting sea foam in the cylinder would be likely to cause any other problems as long as it is spun over with no plug in the hole to clear it out before cranking it. Thanks for the info on the coil packs, I haven't had to work on my ignition system in my 2004 Laredo 4.0 yet and assumed it was individual coil packs like my hemi engines. I should have walked outside and looked at it before replying but didn't have my shoes on yet, LOL. What I don't understand is why a bad or stuck oil ring only smokes at idle, and also when not descending hills in second gear which should pull oil past a bad oil ring at an increased rate if you are in no throttle mode? Normally it would blow a good puff of smoke when you re-apply throttle at the bottom of the hill.strange. I'll go have a look at my 4.0 and see if anything jumps out at me as a possibility.

When people have issues like these we all refer them to YOU as the expert, which you are. I have no idea either Dave this is a tough one for sure and one you have been fighting for a while. I'm sure that you have no codes showing or you would have said so.


Dave I'm going to call about this to a Jeep service manager that I know who has been very helpful to me, if he's in today and ask him. All he can say is IDK like the rest of us lol.

OH, absolutely! I wouldn't want it to hydraulic lock on me! That tends to crack pistons! Any Seafoam that runs into the oil pan can't hurt either. This engine is whistle clean inside because I've used synthetic or synthetic blend oil most of it's life and it runs at least 4 hours at a time on my paper route. No short trips to the grocery store either. I've put 75K on it in 4 years.

Thanks, Fred! I can use some inspiration right now.

Well I called him and he didn't know either and one of the older persons who has worked on 4.0's in the later yrs. the most, is out with the flu.


So much for my bright idea.

I went out and looked at mine and the only thing I can think of short of taking the coil packs off the engine and investigating further is that you have high resistance in the #5 lead causing weak spark at idle that goes away when at higher rpm. Now that, coupled with worn rings, would cause the symptoms you describe.

It's worth looking into the coil pack. Thanks for that. Most mechanics will condemn the coil pack for ANY misfire code. What you say is possible and something I hadn't considered.

Another possibility is a bad valve guide seal (intake, number 5). At idle you have the highest vacuum. As the issue bridges #5 plug, and the smoke goes away when bridged/shorted (no fire to burn oil), this would support looking at the guide seal in #5.

-just my $0.02

Thanks for that, Charlie. Again I failed to mention the first thing I did was replace all the seals, even though it was a new head. I put a Clearwater head on this about 50K miles ago and thought they used NOS seals that were dried out. At least now the oil problem is only in one cylinder. I might pull the valve cover and rocker on #5 and see if the seal got damaged when I installed it. Good Point!

When the head cracked, it only cracked into an exhaust port and I had coolant dripping out of the tailpipe. None in the oil.

I wish I could help you Dave but I can only parrot what Fred said - you're the one we seek out for expert help.

Please pardon the puffery but I'm wowed by the details of this dialog. I've learned more about diagnosing a bothersome engine malady here in three minutes than I have in years! I must say that you three amigos are like the holy trinity of engine troubleshooters! My humble thanks for this.

What I needed was a brain pool of like minded people to blow away the smoke and cobwebs from my mind. And I got just that. Thanks guys!

As far as the internal workings of internal combustion engines are concerned, I've been intrigued with them since I was 14 years old. I knew the basics of the Otto cycle (4-stroke), but really got an awakening when I realized they were controlled by how much air you gave them! I was stunned! So simple!

I actually got started with steam locomotives. At the time, they were about the most powerful machines on the planet. The most dangerous part of a steam engine is the boiler. It's a sensitive beast sort of like an atomic reactor. One mistake and it blows up! Gasoline engines are much more civilized.

Most of my knowledge of engines comes from racing and experimenting with new ideas. My friends and I built a transistor ignition system in high school, but we kept burning out transistors. The GM HEI distributor was the industry's answer to simple breaker-point ignition.

I think my first attack is going to be a coil pack. This one's got 208K on it and it's going to fail someday anyway. What's $70 after all the parts I've already put into this?

Shall I ready the parts cannon? LOL.

I would honestly start with the coil pack. Chalk it up to preventative maintenance at this point. Is it possible a valve seal got a little muffed up in the process?

Keep in mind that when a cylinder isn't firing and fuel is still being supplied, you're going to superheat your cat(s).