Thermostat Test

Testing an Engine Cooling System Thermostat

Inside your car's engine, thousands of controlled explosions called combustion events caused by igniting fuel/air mixture inside the engine generate heat. If this heat is not controlled the engine will over heat and internal damage will occur. These high temperatures are controlled with the help of the cooling system. A cooling system consists of a water pump, thermostat, radiator hose, hose clamps, radiator, radiator cap and coolant. The thermostat is designed stop the flow of coolant through the cooling system while the engine is warming up to operating temperature. An engine needs to operate at a particular heat range to be efficient. 

Once the engine is warm the thermostat will open to allow coolant flow and cool the engine. Most thermostats are designed to open at about 195° F but other temperatures are available for a variety of applications. A thermostat consists of a main housing, a plunger style of valve and a temperature sensitive mercury filled plunger that acts as the sensing and activating device with a return spring. A thermostat maintains engine temperature as is opens and closes throughout the engine operation.

Engine Cooling System Thermostat
Engine Cooling System Thermostat

When a thermostat malfunctions it can stick closed not allowing the coolant to circulate causing the engine to overheat. 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.

Test Procedure: 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 gauge, 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 to quickly through the radiator the coolant will not have time to transfer the heat it has absorbed. This will cause the engine to overheat.

Red and Green Engine Coolant
Red and Green Engine Coolant

In the beginning, water was used for cooling systems as it is the most efficient fluid to absorb and dissipate heat. The disadvantage of using water is that it freezes and will rust. Then water was mixed with ethylene glycol which worked as an antifreeze element. Not only does the freezing point improve so does the boiling point by adding ethylene glycol. Ethylene glycol can be heated up to 250°F before becoming ineffective. The cooling system is pressurized to 18 psi. to increase coolant boiling point which adds 9 degrees Fahrenheit to the boiling point of the coolant. Coolant is also used to avoid corrosion. An engine thermostat is designed to stop the flow of coolant from the engine to the radiator. This temperature sensitive valve is designed to open when the engine has reached operating temperature (190°-198° F).  This operating temperature is used to help facilitate the fuel combustion process.

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 colder longer than necessary. If the thermostat fails "open" it will cause the coolant to continuously flow through the engine creating a diagnostic trouble code (check engine light) will be set. When a thermostat sticks 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 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 replaced.

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. Always drain and refill your cooling system regularly.
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Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-04-11)