Coolant System Diagnosis - possible air lock

2000 XJ 4.0

Replaced all hoses, did a flush, new temp sensor and new thermostat(195).

I get no heat from the heater core and after driving about 5-10 minutes this is happening-Typically it will heat up to 210 stick there for a little(I falsely think it's ok) and then it slowly creeps to 220 and then it it just goes bonkers:

https://www.youtube.com/watch?v=atIfSvqZ7W0

I think the top hose has an air bubble? Should I do the "drill an 1/8" hole" in the thermostat mod?

If the radiator has a removable cap, the cooling system is self-burping. After one or two driving cycles, all the air should be gone. I just replaced a radiator and hoses last week and had no problems.

If the temperature sensor or the thermostat is not genuine Jeep, I would suspect it was poorly made by Chinese children, and should be tested for proper function, or replaced.

I tested the thermostat before install. Temp sensor is good too.

I just did a another heat cycle, I didn't let it overheat. Prior, I did some priming by squeezing the top hose and using the lisle radiator funnel. The top rad hose seems to be full now. I can't confirm it's totally fixed, but seems better.

I'm still having trouble getting heat. I do have the flushing tee installed on the heatercore inlet hose and I let that bleed out too, but strangely (or not-so strangely) the half coming from the thermostat housing is HOT and the half after the tee is cool to the touch. The outlet hose is totally cool too. Seems like I'm not getting any flow after that flushing tee. I will try to bleed it again after the system cools again.

You say you are having trouble getting heat, and the heater core outlet hose is cold ? I would suspect a clogged heater core, or that the fins have rusted off the water pump and you have no coolant flow at low rpm's.

I figured since the heater hoses never got hot, there was zero flow into those hoses. I went for the water pump to rule that out. I was hopeful it was the pump at this point.


Impeller turned ninja star. That being said, the bearing/seal/shaft were still in good working condition after 190k!

New pump installed, all issues went away! Burped the system with ease. Piece of cake.