Hello, I have a 1999 Jeep Cherokee Classic, 4.0L w/ automatic trans, 148,xxx miles, 2WD.

So i'm driving home from work yesterday and out of nowhere get a flashing CEL and a some hard vibration/rumbling from the motor. I pulled over and inspected everything I could but didnt see anything. I decided to try and make it home despite the flashing cel and figured out that the problem ceased at higher RPM, however if the RPM dropped below 1,000 the motor died. At this point the idle is extremely rough when cold, and just kills the motor when warm. The exhaust seems to be pushing out at a higher pressure than normal from the exhaust pipe.

I stopped at autozone and got a Cylinder 3 misfire code, nothing else. After getting home I verified the plugs were firing and checked that the injectors were getting power, both good.

Then came the compression test, which crushed my hopes of an easy fix.
#2 - 170psi
#3 - 0psi
#4 - 165psi
Tried putting oil in the cylinder, no change in pressure, no needle movement at all while turning motor over.

I took the valve cover off and everything looked pretty good. All pushrods seem solid and in place, a few rockers had a little side movement but solid in both pivot directions, valve springs look good.

Stuck a borescope down the spark plug hole and didn't see anything in particular. Both valves look clean and in place. tried to inspect cylinder and piston for holes but couldn't find anything, however I will admit i'm not the most proficient with it so I could have easily missed something.

Put soapy water on the injectors and turned motor over hoping to find some odd leak there, but nothing.

After reassembling from this I started it back up, still a really rough rumbling idle and high pressure from exhaust pipe, however this time i'm dumping white smoke. I'm assuming its from the oil in Cyl3, checked pressure again, still 0psi.

So basically i'm thinking I either have a hole in #3 cylinder or piston, a bent valve, or maybe a unique blown headgasket that is only effecting one cylinder and not leaking coolant.

Anyways I was hoping to get some thoughts on anything I may have missed. Thanks for taking the time to read this.

 

Did the rocker arms for #3 function?

 

Yes both functioned normally. I turned the motor over a few times with the valve cover removed, the pushrods, rockers, and valves all operated smoothly.

 

Pressure has to be going somewhere, can you perform a leak down test?

 

Sure sounds like a valve problem. If you had a hole in a piston I'd think you'd have some very noticable crankcase pressure building up.

I had a similar problem with a "Brand X" car some years ago that had a couple of hundred thousand miles on it. It suddenly started running poorly and a compression check showed 0 psi in one cylinder. I pulled the head to see what was going on and the exhaust valve on the dead cylinder actually had cracked, with a section like a piece of pizza starting to separate from it. Since everything else looked OK I installed a refurbished cylinder head and proceeded to drive it another 100K miles or so.

 

What is the compression on 1,5,6?

 

You can run it with valve cover off and watch #3 rockers. If the rockers and valve stems move then you have a hole in the combustion chamber somewhere

 

Or just pull the coil wire & crank to see the valve problem.