How to Test a Thermostat
Many professional mechanics use one of three simple methods you can apply to learn how to test athermostat. The best part is that you don't need mechanical knowledge or special skills. Depending on the specific method you use, though, you may need one or more simple tools, or none at all. Use one of these techniques to troubleshoot your car thermostat right at home.Method 1: Watching coolant flowWith this method, you don’t need to remove the thermostat from the engine for testing. However, your radiator must have afiller neckand be readily accessible. To prepare for the test, open the hood and make sure the engine is cool. If necessary, wait about an hour or more for the engine’s temperature to drop. Next, set the transmission in Park (on automatic models) or Neutral (on standard models). Then, engage the emergency brakes.Remove the radiator cap from the radiator neck and start the engine. Observe the coolant through the radiator neck. The coolant should not be moving or flowing. This is because thethermostat, at this point, should be in the closed position. If you see the coolant flowing, your thermostat is stuck in the opened position and you need to replace it.Wait for about 10 to 15 minutes for the engine to warm up. Once the engine has reached operating temperature, you should see thecoolant beginning to flowthrough the radiator neck. This is because the thermostat has sensed the high temperature of the coolant in the engine side and the valve has opened, allowing hot coolant to flow towards the radiator and lower-temperature coolant in the radiator to flow towards the engine. If you don't see the coolant flowing, check the temperature gauge on the dashboard of your car. If the needle is reaching the red line, but the coolant is still not moving, shut off the engine. Your thermostat is stuck in the closed position and you need to replace it.Method 2: Probing thermostat operation with a thermometerOn some engine models with a hard to reach radiator cap, you can test the thermostat without watching the coolant through the radiator neck using a simple kitchenprobe thermometer. Make sure the engine is cool and then start the engine. Take a temperature reading on the engine side of the thermostat housing-the component the upper radiator hose connects to on the engine side. Now, take another reading on the upper radiator hose near the thermostat housing. Jot down these two readings on a piece of paper. Leave the engine idling.After 10 or 15 minutes, take another temperature reading on the same two locations. The temperature on both sides of the thermostat housing should have gone up. This is because the hot coolant on the engine side is now beginning to circulate through the upper radiator hose and into the radiator. If there is a significant difference between the temperatures on either side of the thermostat housing on your second reading, your thermostat is stuck in the closed position. If the temperature on both sides of the thermostat housing didn’t rise significantly, compared to your first reading, the coolant is circulating freely throughout the cooling system all the time. In this case, the thermostat is stuck in the open position and you should change it.Method 3: Testing a car thermostat out of the engineIf for some reason you need to remove the thermostat out of the vehicle, you still cantest the thermostat directly. First, check the middle section of the thermostat’s disc. This section should be closed completely. If you see an opening, your thermostat is stuck open and you need to replace it.Boil some water in a kitchen bowl or an empty can of soup. Grab the thermostat with a pair of needle nose pliers and submerge it in the boiling water for about one minute. Pull the thermostat out of the water and watch the middle section of the thermostat disc. The thermostat spring, located on one end of the unit, should have compressed and opened the middle section of the thermostat’s disc. If the disc is still closed, the thermostat is stuck closed and you need to replace it.Any one of these three methods will help you determine the condition of your car thermostat. They are easy to apply and you don't need to invest in special equipment or tools. In a few minutes, you can verify whether you need to replace the thermostat or there’s another problem affecting your engine’s operating temperature.
