What Does the Engine Thermostat Do?

If a car's engine is too hot, it can actually warp and become damaged. But when it's cold, it doesn't run efficiently. So it is important to keep the engine at the right temperature, which is where the thermostat comes in. An engine's thermostat regulates the temperature of the engine by controlling coolant flow.

The coolant does exactly what you imagine—it cools the engine down. The engine thermostat can open and close. When it's open, the coolant flows through, lowering the temperature of the engine; when it's closed, the coolant is blocked until the engine warms up. Engine manufacturers will install a thermostat that opens at the car's operating temperature (usually either 82 or 100°C). While the thermostat controls coolant flow, the radiator's job is to cool the fluid. The flow of coolant does not cycle through the radiator unless it is hot and needs to be cooled.

• When an engine is cold, the radiator fluid is cold, so the thermostat is in the closed position. In this position, coolant will only flow in the engine. It will not be able to leave the engine to cycle through the radiator to be cooled.
• When the engine starts up, and the coolant warms. When the coolant reaches a specific temperature, the thermostat opens. Each thermostat is gauged to open at a specific temperature. Once it opens, the coolant can circulate through to the radiator to be cooled.

The Other Radiator

There's another radiator-type component under the dashboard called the heater core. When the car is warmed up and the heater is turned on, a fan blows air over the heater core. The heat produced by the hot radiator fluid runs through the heater core and is dispersed into the cabin of the car, heating the car's interior.

How to Tell If Your Thermostat Stopped Working

When the thermostat gets stuck and stops working completely, there are several indicators you can check to determine the culprit. See the table below.

Thermostat Symptom Tests and Explanations

Symptom Test	Explanation
Cold engine test: Open hood and remove radiator cap. Start engine. Observe coolant action. It should not be moving.	When an engine is cold, the thermostat should be closed, thus bypassing the radiator.
Warm (not hot!) engine test: Open hood and remove radiator cap. Start engine. Observe coolant action. It should be swirling vigorously.	When an engine is warm, the thermostat is in the open position. This allows the coolant to flow through the radiator causing vigorous movement.
Cold engine test: Start the engine. Time how long it takes the engine to warm up enough to produce heat when the heater is turned on.	If it takes more than five minutes for the heater to produce heat, it is a sign that the thermostat is stuck open, allowing all the coolant to flow all the time. This causes it to take a good deal longer for the engine to warm up.

Location

You can find your car's thermostat by starting at the upper radiator hose and following it to where it enters the top of the engine. The thermostat housing is right where the radiator hose meets the engine housing.

When the thermostat becomes stuck, it is reasonable to simply replace it. Also, when the thermostat housing is removed, the housing gasket should be replaced even if you're just checking it. A housing gasket needs to be able to seal fully, and it won't if it's been used.

Removal and Testing

If you are still unsure whether the thermostat is stuck, perform the following procedure to get an accurate diagnosis.

1. Disconnect the negative battery cable.
2. Drain some of the coolant so the fluid won't pour out when you perform the next step.
3. Remove the upper radiator hose.
4. Remove the thermostat housing.
5. Remove the thermostat.
6. Locate and write down the temperature stamped on the lip of the thermostat.
7. Fill a pot with cold water and a thermometer and place on a stove burner.
8. Place the thermostat into the cold water.
9. Turn the burner on.
10. Watch the thermometer. When the temperature rises to the number you recorded in step #6, the thermostat will start to open. If it does not, or if it doesn't open until reaching a different temperature (hotter usually), then the thermostat is bad and should be replaced. If it opens at the designated temperature, then the thermostat is just fine.

Another Way to Tell If the Thermostat Is Faulty

Helpful Warning

The thermostat is such a vital component to the well-being of your vehicle, yet it is so cheap. Replacing it is far less costly than repairing a warped engine. A warped engine must be removed and either resurfaced, if the damage isn't terribly bad, or completely replaced.

Sometimes car owners remove but do not replace their car thermostat. This is not a good idea, because a cold engine does not function efficiently and a constant, slow warm-up will take years off of the engine's life.