I have a 2000 XJ that I swapped an AX15 into. it has 160K miles. It ran fine for a few months after the swap, so i don't know if that's the problem. Also, before I bought it, the radiator broke and it blew the head gasket. The PO had a shop work on it, and judging from how the thermostat cover thing started leaking almost immediately (i forget what it's called, the thing with the temp sensor and radiator hoses). They didn't use a real gasket, just orange goo stuff. gasket maker or something. Anyways, i think there's some problem with the cyl head. I checked the compression for each cyl, and they range from 70 to 105 psi. I seriously doubt the rings are bad at only 160K miles. modern cars do just wear out that quickly, especially not an engine known for its longevity, like the 4.0L

Any suggestions? could it be the rings? does it need a rebuild? cylinder head work?

oh yeah, another thing - it just happened one day. no slow loss of power over time, nothing like that. I took it on the highway, it wouldn't go over about 45, so i immediately got off the highway and drove it home. Also, if I drive it for a while, and then turn it off, it won't come back on. If it's cold, it starts right up. Well, it cranks a few times first, but it's always done that.

Normal compression should be over atleast 120 psi a healthy engine is around the 140 ish area so if your getting below 100 id do a cylinder leak down an figure where its leaking from

+1. Drop about a spoonful of oil into each cylinder right before running the compression test. The oil will work its way down and try to seal the rings. If results improve, then its rings. If not, problem is overhead. Carbon buildup or bad valve springs possibly. Though I do not believe this is a "leakdown test", pretty sure thats to do with the cooling system.

Cylinder leak down is not for cooling its for the head an rings you have to get the pistion your testing at tdc an apply regulated shop air no more then 100 psi threw the spark plug an is it holds the air it good if not just listen to where the air is leaking from if you hear it from the dip stick or oil fill cap is the rings if you hear it from intake its an intake valve if you hear it from exhaust its an exhaust valve an well if it bubbles in the coolant its a head gasket or toasted head

Ah, I haven't heard it used like that before. I was only familiar with that test being used to troubleshoot a bad head gasket by watching for bubbles at the rad cap.

Ya thats the easy way this way is a little more o ya an i forgot to mention previously you gotta take all plug's out for the test an well if the air leaks into another plug (cylinder) could be block or head