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Overheating when heater is on

I had an overheating problem about 4 years ago. During the summer and before taking it to a mechanic, I’d run the heater to cool down the engine - worked fine.

The mechanic I brought it to seemed to fix the problem - no need to run the heater.

However, when winter arrived and I turned on the heater, the engine started overheating! To keep warm, I’d run the heater and when I saw the temperature gauge climb, I’d turn it off.

Strangely enough, that problem went away for about a year, but it’s back now.

Any insights would be appreciated. If it matters, my truck is a 1989 4wd pickup, fuel-injected, 22R-E engine.



I would try a new thermostat.

You might check if there’s air trapped in the cooling system.

The coolant to the heater core is controlled by a heater control valve. Since the heater core is the highest point in the cooling system, air can collect in the heater core. Then when heat is asked for, the control valve is opened and coolant is pumped into the heater core pushing the air out. If this air reaches the thermostat it can cause the thermostat to close because thermostats aren’t designed to detect air temperature. Or the air may reach the water pump causing the coolant flow to stall. Either one of these will cause the engine to overheat when the heater is used.

To make sure there’s no air in the cooling system try this. Get the engine up to operating temperature. With the engine idling, and with the heater turned on to open the valve, losen the upper radiator hose clamp. Take a small flat bladed screwdriver and insert it between the upper radiator hose and the hose neck on the radiator. Allow the engine to continue to idle until all the air is purged from the cooling system and until nothing but coolant comes out. Remove the screwdriver and retighten the hose clamp. Check the coolant level after the engine has cooled off.


I never have personally experienced a situation where an overheat condition was mitigated by opening the heater water valve BUT I do not have enough toes and fingers to count the number of people who report that this works. What would the parameters of the failure have too be for opening the heater water valve being able too bring and overheating car back down too a temperature where you could continue too safely drive?