Stuck Thermostat

STUCK THERMOSTAT
Stuck Open - The thermostat fails to fully close when the coolant temperature is below the opening point. This allows coolant flow through the radiator while the engine is cold and prolongs warm up. If the thermostat is stuck fully open, it may prevent the engine from reaching normal operating temperature.

Stuck Closed - The thermostat fails to open during engine warm-up. The engine warms up normally, however when the engine reaches normal operating temperature it overheats rapidly. All coolant flow to the radiator is blocked.

SYMPTOMS
Thermostat Stuck Open
- Engine temperature fails to reach, or requires an unusually long time to reach normal operating temperature.
- During cold weather the heater output temperature may not warm up.
- The upper radiator hose begins to warm up as soon as the engine is started.

NOTE: With a cold engine the upper hose should not begin to warm up until the engine has reached normal operating temperature and the thermostat has opened.

Thermostat Stuck Shut
- Engine temperature warms up normally but upon reaching normal operating temperature the engine overheats rapidly.
- If the radiator cap has been left off (for measuring coolant temperature) very hot coolant may be rapidly expelled from radiator with little or no warning.



WARNING: Never remove the radiator cap when the engine is warm. Severe personal injury may result.




- The upper radiator hose remains cool, even while the engine is overheating.

WARNING: Use a surface thermometer or infrared temperature gun to determine hose temperature. Do not attempt the feel the temperature by hand, this may result in serious personal injury.

VERIFYING THE PROBLEM
Remove and inspect the thermostat:
- Verify the thermostat is fully shut when removed from the vehicle, by holding the thermostat upwards towards a light, check that light is not visible between the seat and valve. If any light is visible the thermostat should be replaced.




- If the thermostat is not visibly damaged, place it in a pot of water and heat the water until it reaches the opening point. If the thermostat does not open at the required temperature it should be replaced.

For Service and Repair information: Thermostat