One of the most common reasons a car or truck engine overheats is a faulty thermostat. The thermostat closes when the engine is cold and opens as the engine warms up to allow the coolant to circulate through the engine. Follow these steps to test a cooling system thermostat.
Locate the engine's thermostat, normally at the front of the engine, by tracing the water hoses from the radiator to the engine. One hose goes from the radiator into the thermostat housing on one side, and the other hose goes out of the housing back to the radiator on the other side.
Open the radiator cap and verify that the radiator is full. Replace the radiator cap and start the engine.
Turn the vehicle's heater on high. Allow the engine to warm up for a few minutes. Be careful not to let the engine overheat.
Feel each of the hoses leading to and from the thermostat housing. If both hoses are hot and the heater is pushing out hot air, the thermostat is functioning properly. If not, the thermostat may be stuck, and you'll need to remove the thermostat from the engine to perform the next test.
Turn off the engine and allow the engine to cool.
Remove the bolts holding the thermostat cap in place.
Lift off the housing cap to expose the thermostat. Pull the thermostat out of the housing with your fingers. Take careful note of which way it sits so that you can reinstall it or its replacement in the same position.
Put the thermostat in a pan of water on the stove, and turn the burner on high.
Monitor the water temperature with an oven thermometer. The thermostat should open at about 195 degrees F if it is functioning properly.
Replace a faulty thermostat. Reinstall it if it opens and closes properly, and look for another problem in the cooling system.
