Overheating after thermostat change

MR2HACKER

MEMBER
1991 TOYOTA MR2
2.2L
4 CYL
2WD
MANUAL
223,000 MILES

I bought the vehicle in the summer time and have been driving it everyday since the end of June I’ve noticed I never reached normal operating temp. Now in the winter I don’t like running It in the morning so cold. So I decided to install a thermostat. I wasn’t surprised to see there wasn’t one installed in the first place, bought a new Murray thermostat, tested it in hot water and watched the needle pop open at 180*F. Installed the thermostat with the jiggle valve at the 6 o’clock position. Ran the vehicle and burped the system using the 3 bleeder valves until all air was out and only coolant was flowing. Car now will overheat while I let it sit at idle and remain getting hot as I drive. Did a compression test and a block monoxide test, determined the head gasket to be good. Water pump is located behind the timing cover so I haven’t been able to check that. However with the thermostat installed, I have one side of the system hot, and the other cold. With the thermostat out, both sides are hot. With this new findings, I purchased another thermostat, tested it in water and receiving the same results.

1 Reply

MOTOR MASTER

MECHANIC
279 POSTS

Hello my name is Dave.

It sounds to me like you are probably getting an air pocket behind the thermostat, this will prevent coolant flow especially with the long pipes running to the radiator on your car. When you remove the thermostat the coolant flows more freely making purging the air from the system a lot easier. The manufacturer instructions for your car says to open both the radiator and heater air drain valves and I am attaching some diagrams to help guide you with the proper purging procedure which is performed with the engine NOT running. I am also copying and pasting the actual instructions (including misspellings) which requires the use of service hoses as you will see in the diagram. I hope this helps you resolve your issue, please keep us updated on your progress and thank you for using 2CarPros!

NOTICE: If the water level in the air drain service hose is clearly lower, air removal has been incompleted so check if the service hose is crushed or bent, the perform step (c).
F. When the coolant level in the air drain service hoses stops dropping, close the air drain plugs of the radiator and heater.
G. Fasten the radiator cap to the first stop point.

NOTICE: Do not tighten the radiator cap completely (to the second stop point).

H. Start the engine and run at fast idle for approx. 3 minutes, and stop the engine. (With heater set to hot with fan on high)

i. Add more coolant and repeat steps (h) and (i).

HINT: By performing steps (h) and (i), 0.2 - 0.5 liters (0.2 - 0.5 US qts) can normally be added.

J. Check that the coolant level in the water filler has not dropped.
If the coolant level has dropped, add coolant and repeat the steps from (g) onward.

Images (Click to make bigger)

Overheating after thermostat change

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