My wife drives a 2000 xj that I maintain, tonight she called with an issue. A little background first. 4.0 with an auto, 131K. I changed the oil about 2 weeks ago, valvoline 10w30. October 2011 it got a new head gasket and all the crap that goes along with that (3 angle valve job, magnaflux the head for cracks, new valves, new valve guides and seals, while i was in there a new water pump, thermostat, power steering pump, and a flush the cooling system since oil was in the coolant).
Tonight she calls and says the car is overheating, she turned on the heater to help cool it off but the heater is blowing cold air and intermittently hot. I checked it out and there is plenty of coolant, the temp is over 210 but not into the red. The oil pressure is down around 10psi, this motor never has less than 40 psi, hot cold makes no difference. I am thinking cracked head between 3 and 4 but I don't have the time or money to pull the head and have it magnafluxed. What am I overlooking? I really don't think it is the head but not being really well versed in 4.0 I don't know what else it could be. If it is the head is there an easy way to check short of pulling the head?

Thanks in advance for any help you guys can offer

 

Why wasn't the head replaced previously, did it overheat then as well? All the symptoms you posted plus no real oil pressure sounds like the engine is done for good now. Rent a cooling system pressure tester and pump it up.

A cracked cylinder head will rarely allow oil into the coolant but you will almost always have coolant in the oil.

 

I didn't replace the head previously because the head magnafluxed fine, no cracks. It did not overheat then, but there was oil in the coolant. That was the only reason to replace the head gasket (jeep still ran fine) The head went to the machine shop to be decked, have new seals and valves put in and the magnaflux was done because why would you do any work on a used head without knowing its condition first? and secondarily if you have the head out why not do all the easy work?

I let the motor cool down for an hour and pulled the thermostat on a hunch. There is no coolant behind the thermostat, the oil looks good, the coolant looks good. I started the motor back up and the oil pressure is back to its normal 40 psi. I opened up the radiator to see if the coolant was moving. It's not. I checked the coolant return hose, it is not warm after the engine was running for a few minutes and the temp was at 150 with the thermostat out. I am betting on a shot water pump. I think the low oil pressure was merely because it was too warm and the viscosity decreased. Tomorrow I will replace the pump and thermostat and update.

 

Best of luck man. Viscosity aside, pressure is caused by restriction (clearance) or lack there of.