Test to Verify a Stuck Thermostat in a Car

The thermostat in your engine regulates the flow of coolant. When the engine is cold and the thermostat is closed, coolant flow is restricted. This helps your engine warm up faster. As the engine approaches operating temperature, the thermostat begins to open, allowing coolant to flow. Thermostats are rated to fully open at a specified temperature, for instance, 205 degrees Fahrenheit. A stuck thermostat can cause problems ranging from an engine that takes a long time to warm up, to overheating. There are a couple of simple tests you can perform to check for a stuck thermostat.

Instructions

1

Park the vehicle on a level surface and set the parking brake.

2

Remove the radiator cap from a cold engine. Start the engine and look at the coolant in the radiator. If you see the coolant moving, the thermostat is probably stuck open. An engine with an open thermostat will take a long time to warm up. Replace the cap once you are done.

3

Allow the engine to warm up. Check the temperature of the upper radiator hose. If the hose is not hot and the engine begins to overheat, the thermostat is probably stuck closed.

4

Allow the engine to warm up. If the engine is running hot, feel the upper radiator hose. Try to squeeze it. If it is hot and very hard, the engine has probably overheated at some point. A hard hose usually indicates a blown head gasket that is allowing pressurized gas from the combustion chamber to enter the cooling system. A repair shop can do a test for the presence of gasses in the cooling system.

5

Check the thermostat by removing it from a cold engine. You will immediately see if it is stuck open. Check to see if it is stuck closed by filling a pan with water far enough so you can submerge the thermostat in it. Place the pan of a stove and heat the water. The thermostat should open as the water approaches boiling. In either instance you should replace the thermostat. In fact, since you already have it apart, replace it anyway.

6

Allow the engine to cool. Remove the radiator cap. Look for a milky substance in the coolant or on the cap. This indicates motor oil in the cooling system. Also check the oil filler cap and dipstick for the milky substance. This indicates coolant in the motor oil.