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 overheat and internal damage can 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 to control 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 to maintain a particular temperature. 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 with return spring and a temperature sensitive wax filled plunger that acts as the sensing and activating device. A thermostat maintains engine temperature as is opens and closes throughout the engine operation.

Engine Thermostat
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.

Engine Cooling System
Typical Engine Cooling System

Engine coolant is used to transfer heat from the engine to the radiator by the cooling system. The radiator removes heat from the coolant by forcing air through the radiator fins. Without coolant your engine will overheat and if left unattended severe engine damage will occur. Coolant colors can vary from green, orange, blue and yellow each having their own protective properties. (Note: coolant and antifreeze refer to the same product, in below freezing, coolant lowers the freeze point hence the name anti-freeze and in warm weather coolant helps raise the boiling point, "coolant")

Engine Coolant
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.

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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 2013-11-19)