98TJ 4.0L with no compression in cylinder 3 - long

No expert here, but with the gas smell coming out of the exhaust and no gas in the oil, it you might also look into the possibility of an ignition problem as opposed to a valve problem.

Not very likely as he pointed out:

Ignition won't mean diddly if there ain't no compression. The gas smell was likely unburnt fuel because of the cylinder not firing (because of no compression)

How could this be true? Doesn't checking rings require compression of some sort?

Sounds like valve issues to me. Sometimes hardware dies regardless of how well you maintain it. Especially since you've lost only one cylinder, it sounds like some particular part of it's actuation has crapped out.

Eh, oil's cheaper than new parts (and the hassles). I stick with a 3-5k range. If you're doing nothing more than daily driving (not off-road or towing a trailer) and we're not talking searing summertime heat then it's reasonable to stretch it a bit. But as engines age they do wear and the oil will get fouled faster. So on older engines it's a good idea to keep an eye on the oil changes.

-Bill Kearney

I'm no expert either, burt how would that explain the zero compression in cyl #3?

I'm thinking the exhause valve is stuck open or not fully seating closed. All the mixture going into that cylinder is being blown out before it gets a spark.

Here's what I would do: Drain the oil again. Pull the plugs and the valve cover Have an assistant turn the engine over and watch the valves. (With no oil it will save from being messy) Compare the valves in cyl 3 to another good cylinder. Pay special attention to #3's exhaust.

-Fred W

You make two statements that are contradictory here. First, #3 had zero compression. Next, they said the wet test was fine. Something doesn't compute - either just adding oil to the cylinder miraculously restored the compression or there was still no compression which means that they couldn't tell squat about the rings anyway. Makes me wonder about what they were looking at.

Last time I saw this kind of thing I had a stuck valve. Time before that, a lifter had failed. With the valve cover off, you should see both rockers for #3 moving (good lifter). Put your finger on the valve side of the rocker and press down hard while it's turning over - a stuck valve will not push back up near as hard. Since you seem to be pumping gas thru the tailpipe, I'd be looking at a stuck/burned exhaust valve. A stuck/burned intake would be sending pressure back thru the intake and you would feel (and hear) that at the throttle body.

With zero compressi> This morning on the way to work I lost power while doing about 30mph

I also think it's a valve problem. I would put some Marvel in the cylinder and put the plug back in and let it run and see what happens. I don't think a valve can instantly burn, it could brake off and the boroscope should have seen that. So that leaves carbon holding it open or a sticky valve. Also I agree With Bill on the K&N filters. During Mt St Helens years ago only those with k&n filters didn't have filter clogging problems. Why do you think that is? I'll tell you what I think, the dirt just went on into the engines.

JoBo

It doesn't. But a bad diagnostic might. Has any other shop checked the compression on Cylinder 3?

Thanks for all the comments.

I took the Jeep to a local shop ran by an old fellow and his two sons. They were baffled by the diagnostics too but seemed more competent than the mechanic at the 5-Star dealer. Upon removing the valve cover they found that the valve spring on the exhaust valve was broken. They confirmed that the valve cover had indeed been removed by the dealer so the 5-Star mechanic simply missed it. They recommended replacing all the springs since it appeared to be a fatigue failure. With that they also replaced all the spark plugs since they had 85k on the factory originals anyway. My total bill from the local shop came in at just over $400. A far cry from the total of removing/replacing the cylinder head.

I called the GM of the dealership and explained the situation. Curiously he asked if the shop that did the work could confirm that the valve cover had been off. Sounds like he questioned the integrity of his own service people. He's refunding the bill for the work they did and throwing some extras in to convince me to give them another shot the next time either of my vehicles needs work. Not sure how effective that will be.

I'm aware of the downsides of running with a K&N air filter. I drove this Jeep for 70k miles in Arizona before moving to Minnesota in 2002. The amount of damage done by all the suspended particles in the air while off-roading in AZ will likely dwarf anything I could ever do here... unless I hydrolock it while bogging and then the air filter won't matter anyway.

Thanks again. Hope everyone has a good Jeep'n weekend.

mc

Wow.....

That really has to make you wonder eh? Once they started tearing down the head I wonder if they would have been honest enough to stop when they spotted the broken spring or if they just would have taken you like they 'sure' seemed intent on doing. LOL! Missed a busted valve spring on the dead cylinder in question while 'looking' for a broken spring.... Ya Right....

There is 'no' other reason to open the cover except to inspect the valve mechanics in question.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Mike Cagle wrote:

Mike,

Glad to hear you found the problem and got it fixed so inexpensively. It's also gratifying that the symproms you described did in fact add up to a stuck open exhaust valve. ...and it's a *really*& good thing you didn't listen to the RC (resident curmudgeon) and have a total rebuild done!!

Agreed. I'd say the "old fellow and his sons" have done a bunch more to earn your future business than the stealer and his inept gang of thieves.

Keep the rubber side down...

-Fred W

Agreed, I was voicing the possibility the test may have been bad.

I had the wife pick up six quarts of Mobil1 tonight and a new oil filter while she was out. Drained the cheapo oil with the ATF I added to it. Replaced the filter and put the six new quarts in. Good as new. The old fellow said it was really clean inside when they took the cover off. No sludge at all. With all the stuff I put in it to fix a non-existent carbon problem it better be. Tks again.

mc

98 TJ Sport 85k miles, all new valve springs and plugs