Engine Thermostat When a thermostat malfunctions it can stick in the closed position not allowing the coolant to circulate, causing the engine to overheat. This will cause the vehicle to overheat in a very short amount of time (about 5 minutes). Or the thermostat could stick open causing the engine to run too cold. In this case the service or check engine soon light could illuminate, followed by a trouble code. To test a thermostat remove unit. Prepare a pan of water deep enough to cover the thermostat completely. Next install a temperature gauge into the water along with the thermostat. A cooking thermometer works well for this. Next, start heating the water while watching the thermometer, the thermostat should remain closed until the water reaches 190° at this point the thermostat should start opening and be completely open at about 195°. If the thermostat stays closed through the boiling point the thermostat has failed and needs replacing. If the thermostat is stuck open or broken it has failed and needs replacing. Never run an engine without a thermostat because the thermostat works as a system flow regulator as well. What this means is the thermostat has a specific opening that regulates the flow through the cooling system. If the coolant is allowed to flow too quickly through the radiator the coolant will not have time to transfer the heat it has absorbed. This will cause the engine to overheat.
Typical Engine Cooling System
Red and Green Engine Coolant When a thermostat fails it will either stop the coolant flow at operating temperature "stick closed" or fail to stop the coolant flow causing the engine to run cold longer than necessary. If the thermostat fails "open" it will cause the coolant to continuously flow through the engine causing a diagnostic trouble code (check engine light) to be set by the computer. When a thermostat sticks closed it will cause the engine to overheat quickly, usually within 5 to 15 minutes of operation. To check for either of these conditions, drain coolant and remove the thermostat (under the thermostat housing) if you are unsure of the location of the thermostat on your engine consult a car repair manual. Once you have removed the thermostat inspect the condition of the main body check for any cracks or broken pieces and check the valve to make sure it is closed. If the valve is open the thermostat has failed and needs to be replaced. To check the operation of the thermostat prepare a pot of water on the stove deep enough to cover the thermostat completely. Place the thermostat in the pot of water, turn the stove on a medium-high flame, the thermostat should open right before the water comes to a boil. If the water has boiled and the thermostat valve is still closed the thermostat has failed and needs to be placed. Basic Checks Low levels of coolant can lead to problems related to engine overheating. Always maintain proper levels of coolant in the overflow coolant reservoir tank. ONLY! Check the radiator when the engine is cold. Only add antifreeze and reinstall the cap properly. If further assistance is needed, our certified car repair technicians are ready to answer your car questions. Related Car Repair Information