I own a 1997 jeep grand cherokee Laredo 4.0 that isnt running, it cranks but wont start, and I did a compression test on all 6 cylinders and I got about 30 psi for each. I assumed it was my head gasket so I started tearing stuff apart, but my head gasket is fine. Does anyone have any ideas to of what could be going on?

 

Yeah, valves or rings. You should have done some more testing before you tore it apart. Now you'll have to put it back together and do a cylinder leak-down test or tear the head apart and check the condition of the valves and pour some really light oil on top of the pistons to see if they leak into the pan. I would recommend diesel because it cleans out easily. What do the cylinder walls look like? Do you have oil pressure while cranking? The timing chain may have jumped so the intake valves are open during compression.

A blown head gasket will usually show low compression on 1 cylinder or a pair next to each other, not all the same. Was there any coolant in the oil?

When diagnosing an engine, don't assume anything. By pulling the head, you're made more work for yourself. When doing a compression test, you should disable the ignition and fuel pump and block the throttle wide open.

 

My valves and ring are fine, and I did do the compression test correctly. Cylinder walls look. Everything looks fine, I even had my shop teacher look it over. We did a bunch of test and his guess was the head gaskets, but turns out that isn't the case. My next guess would be that my chain jumped timing? How would I check something like that

 

With the engine reassembled, you can check to see if both valves are closed when a piston is at TDC compression. Otherwise, tear the front end off (water pump and damper pulley) and check the timing marks on the chain and gears. If the chain is stretched at all, it might be a good idea to replace it and the gears anyway as long as you're that far into it.

Sorry I dumped on you, but you never said you had the shop teacher look at it. Let us know what you find. It may help others. This now has ME intrigued!

IDK if the 4.0 is an "interference" engine or not, but if the valves hit the pistons, they could be bent. If that happened, there should be marks on the pistons. I'm "assuming" you already checked that.

 

Did you pull pistons to verify this or did you look at all of the top rings in their bore? While they may look fine, there is a second ring you can't see and if the rings were fine you would have compression. It's entirely possible they are stuck in the ring grooves and not expanding with the taper in the cylinder, thus allowing cylinder pressure to escape. The 30 psi you are getting is what gets trapped at the top third of the stroke where taper is the least prevalent and the rings are able to seal up against the cylinder again. You don't have to have scored cylinder walls to get taper. Its a natural by product of use.

How many miles are on this engine and how was the carbon buildup on the piston tops?