Tell if Your Car's Thermostat Is Stuck Closed

The thermostat in your car is responsible for controlling the amount of coolant released into your engine. When your car first starts up, the thermostat is closed and does not allow coolant into the engine until it is warmed up. The thermostat should open and allow coolant in once the engine is adequately heated. Sometimes the thermostat fails and remains closed. This article will discuss how to tell if your car's thermostat is stuck closed.

Steps

Testing at Home

  1. Check your temperature gauge in the car. A stuck thermostat might be to blame if your gauge marker nears the red danger zone within five to 15 minutes.
  2. Turn off your car and allow it to cool before opening the hood. Unhook your coolant reservoir if you can undo it and pour the antifreeze into a container. Locate your radiator and remove the cap.
  3. Unfasten the radiator drain cock located near the bottom of the radiator and allow some of the coolant to drain out into a container with a lid until the level is below the upper radiator hose.
    • You might not need to drain the radiator, depending on the amount your reservoir holds. The amount drained is roughly 1 to 2 qt. (4 to 8 cups). You can reuse this coolant if it is brand new. Otherwise, replace it with new coolant.
  4. Locate your thermostat. The thermostat housing is usually under the upper radiator hose. Unbolt the housing and remove the thermostat, using a screwdriver and pliers. Take the thermostat inside. There will generally be a number somewhere on the body, giving it's opening temperature usually between 160 to 190°F or 70 to 90°C. Otherwise you will need to check the Workshop Manual for the car.
  5. Fill a pan with water and place the thermostat into the water until it's completely submerged. Ensure that the part does not touch the bottom of the pan.
  6. Start heating the water and place a cooking thermometer into the water. Check the temperature frequently while monitoring the thermostat.
    • The thermostat should remain closed until it just about reaches the thermostat-opening temperature designated on the body (or in the Workshop Manual). At this temperature, you should see the thermostat begin to open.
    • The part should be completely open when the water reaches approximately 10 degrees above the reading on the thermostat. If the thermostat is still closed at this point, it needs to replaced.

Testing On-the-Road

  1. Look to see if your temperature gauge is nearing the danger point. This tends to happen quickly, so turn off your car as soon as you notice it.
  2. Open the hood of your car as long as it is not too hot and locate the radiator hoses. If the hood is too hot to touch, wait at least 10 minutes before opening it.
  3. Touch the top radiator hose carefully. The hose can be very hot, so only touch it for a few seconds with your fingertips. Repeat for the bottom radiator hose.
    • Both radiator hoses should be very warm to the touch. If one hose is very hot and the other one feels cool or even cold to the touch, the thermostat is likely stuck in the closed position.

Using the Squeeze Test

  1. Start your car and let it sit idling for a while. Or, take it for a short drive to warm up the fluid in the radiator.
  2. Open the hood, with the engine still running. Put on an oven mitt or a heat-resistant glove. Find the top radiator hose (the one that connects to the thermostat housing).
  3. Squeeze the hose near the middle, as if you were trying to squeeze it shut.
  4. Release it, and you should feel a surge as the water comes through. If it is too firm to squeeze the thermostat is stuck closed.

Tips

  • Some newer cars do not come equipped with an engine temperature gauge, only a warning light. When the warning light comes on in these cars, find a safe place and turn off your car immediately. Your vehicle is already overheating.
  • If you do not feel you can diagnose the thermostat yourself, then contact a qualified mechanic to check it out.

Warnings

  • Always put coolant into containers that can be sealed. Coolant smells very sweet, but is toxic to humans and animals in very small doses.
  • Never try and open your radiator cap when your engine is hot.
  • Touch your vehicle's engine with caution. It can be extremely hot and can cause severe burns. If the engine appears to be smoking, allow it to cool more before touching it.

Things You'll Need

  • Container with lid
  • Pliers
  • Screwdriver
  • Pan
  • Thermometer

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Sources and Citations

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