The Thermostat May Be Stuck

If a car’s engine is running too hot, it can actually warp and become damaged. But when it’s cold, it doesn’t run at its optimum. 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 keeps the engine cool. It does this by absorbing heat from the engine as it passes through it. The radiator allows wind to flow over it to remove the heat from the radiator fluid. When there is not enough wind to blow across the radiator for cooling, such as idling in traffic, the engine fan blows across the radiator to remove the heat.

The engine thermostat is made to open and close. This is how it regulates the flow of coolant. In the open position, the coolant flows through, absorbing the heat in the engine; when it’s closed, the coolant is not allowed to flow through the entire engine. Thus, the coolant, restricted in its flow, collects heat faster. When the coolant reaches a specific, predetermined temperature, then the thermostat will begin to open. This allows more coolant to flow through the system to keep the temperature of the coolant at that predetermined temperature.

Engine manufacturers will install a thermostat that opens at the car’s optimal running temperature which is, usually, either 180 or 212°F (82 or 100°C). While the thermostat controls coolant flow, the radiator’s job is to cool the fluid.

  • 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 has started and run for a few minutes, the thermostat starts to open allowing more coolant to flow through the engine absorbing the heat. Thermostats can be purchased with various maximum temperatures.

There’s another radiator-type component in your car located under the dashboard. It’s 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 it stops working properly. There are several indicators you can check to determine if your thermostat has gone out.

  • COLD engine test: At the radiator, remove radiator cap. Start the engine. The coolant should not be moving (just a little jiggling). Explained: 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. Explained: 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. Explained: 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.
A very simplified animation of the thermostat in action.

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. It looks like half of a metal baseball.

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.

The thermostat and housing together cost less than $20. Keep reading, however, to find out how to be sure that your thermostat is the problem.

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 (a gallon should be enough) 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.

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.

Be safe and replace!

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