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Over-heating with no heat in the cab 01 Jeep Gc

01 Jeep Grand Cherokee. I just changed the water pump and the thermostat on the Jeep but the last two days when I drove it to work, the heater was blowing cold air and the engine was getting hot, almost to the red. What do you think that I should look into about getting the problem fixed>?

There may be air still trapped in the cooling system after replacing the water pump.

To purge any air out of the cooling system, with the engine at operating temperature and idling, loosen the upper radiator hose clamp and insert a small screwdriver in between the radiator hose and the hose neck on the radiator. Allow the engine to continue to idle until all the air is purged out of the cooling system and there’s constant heat coming out of the vents.


My wild guess is that you didn’t purge the air out of the system properly.
Did you have the heater ON when you purged the air?
Did you fill it, warm the engine up, and then fill it again?
Was the radiator cap at the cooling system’s highest point?

It’s a secondary guess that the water pump is sitting in air spinning uselessly known as “cavitating”. Have you checked for proper pump flow?

My last guess would be that the T-stat is in backwards and acting as a block against the coolant flow. On some vehicles that can be easily done. You may want to double check the installation.

A bad head gasket is leaking combustion products into the cooling system.

If you changed the pump, then it’s air. Make sure to run the heater while you bleed it.

If there was no overheating before, either the new water pump is defective, the new thermostat is the wrong one, or defective or (as mentioned above) is installed backward or upside down, or there’s air in the cooling system.

If it still overheats after bleeding the remaining air out, see if you can figure out a way to tell if the coolant flow into the top of the radiator is robust. When the thermostat opens, the flow into the top of the radiator goes from a trickle to quite a noticeable volume flow, in just a short amount of time.

If you were having overheating problems and the pump and thermostat replacements were intended to fix that symptom, then you may have a pressure leak somewhere in the cooling system or you have a head gasket problem. You might try replacing the radiator cap. Or have the cooling system pressure tested. There are chemical tests (available at auto parts store sometimes) to determine if there is exhaust gas getting into the coolant, the sign of a head gasket problem.

Hopefully the problem is due to just a stuck thermostat. If you are sure that any air is purged out of the cooling system then I would try changing that out. If that doesn’t work then I would have to suspect a head gasket leak. Checking for exhaust gases in the coolant would verify that issue.