How to 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 heats up, the thermostat closes 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

Test at Home.

1. 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 5 to 15 minutes.

   
   
2. 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. 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. 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.
5. 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. 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 about 190 ºF (88 ºC). At this temperature, you should see the thermostat begin to open.
   • The part should be completely open when the water reaches 195 ºF (90.6 ºC). If the thermostat is still closed at this point, it needs to replaced.

On-the-Road Test

1. 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. 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. 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.

   
   

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.

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.
• 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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