I did a compression test on my 98 4.0 yesterday just to see how everything was doing. The readings (psi) that follow are the highest of 3 tests on a dry, cold engine;
1- 155
2- 161
3- 170
4- 161
5- 165
6- 174

In reading other threads, the pressures should be between 120-150psi. Why would mine be higher? The engine isn't even close to new with 200k miles. I'm not concerned about the numbers, just curious what people think.

 

For what it's worth, the FSM says to rotate the engine through 3 compression cycles then take the reading(s). If you let it crank multiple times the readings will get higher, then peak out. Your readings look like that.


Carry on.

 

That would do it, I cranked 5 cycles. I didn't have a FSM to reference, wish I did. Any idea why they say to crank 3 cycles?

 

Not sure with Jeep, but with GM it's to prevent wearing out the starter, draining the battery, and if an engine can't build up compression in 3 cycles, it won't build it up in 5 or 10

 

2000 XJ Sport 4x4. Something has blown and mixed coolant into the oil and I assumed a head issue or gasket but I too had slightly higher numbers but with only one that was really higher and one slightly lower:
6. 155
5. 152
4. 175
3. 140
2. 150
1. 150
Cold engine, 3 Cycles, dry, full throttle, all spark plugs removed. Would my issue be 175 and/or 140, between, or potentially something else? Looking for ideas as I have not yet taken off the valve cover or head yet, only done this compression test and drained/flushed out the tainted oil/coolant so far.

 

Are you familiar with the 0331 head defect? If not, google it. It's a pretty common issue on 00-01 XJ's and should be checked for immediately at the first sign of coolant in the oil.

I'd say that your compression is probably good - maybe repeat the test again. Could be an issue with the gauge or something like that - is it an el cheapo/loaner tool? If CCKen felt that the comparable situation was no worries, then no worries - that guy was/is legendary among the Jeep forums.

If you caught the coolant in the oil quickly (as it appears you have) then you're in very good shape - order yourself a Clearwater replacement head, put it on keep on Jeepin'!

BTW - Welcome to the forum!

 

That's what I am hoping is the issue, I have already checked on Clearwater and planned on buying a replacement head. I'm just hoping it isn't anything worse. Yes everywhere I go I hear of the issues with the 0331. I have an Innova 3612 so I guess that is a cheapie but I did three pressure checks and all came out about the same on all cylinders. Thanks for your reply!

 

Maybe someone smarter than I has a more informed answer.

Possibly gunk in the plug cavity or something like that is all I can think of.
I think the crack is between 3+4 so possibly that is factoring in the readings...