I have a 95 xj, 4.0, 4x4, auto. I have replaced everything in the cooling system except for the waterpump. I did a compression check today to see if I had a blown head gasket. All cylinders read 120psi except one, it read 100psi. Does this mean I have a blown head gasket or a bad cylinder? what is good compression? Why is my motor still overheating? I even went as far as to get a performance 2row aluminum radiator and wire my auxillary fan to a toggle switch. Any help would be great Im about to just buy another motor.

 

Well in general you want all the cylinders to be within 10% of each other. If your peak is 120, you should be within 10% of that for the most part. 100psi is more than that, but not a huge amount. Retest that cylinder just in case the connector leaked, if it's still low then you might have a problem. If putting a little oil (cap full) into the cylinder makes compression go back up then it's likely rings and not a head gasket.

Are you loosing coolant? Sweet-smelling exhaust smoke? Water in the oil? Oil in the water?

If you've replaced everything but the water pump and it's still not right, I'd consider checking that water pump.

 

Spec is 150psi for a good engine. You may have a valve leaking or worn rings. A leak down test would helpfind out where the problem is.