How to Test if a Thermostat is Good or Bad?

[FelixMarie]

Check whether its performance is good and its operation is reliable. Below are the relevant methods for testing a thermostat: 1. When the coolant temperature is high, remove the rubber hose from the cylinder head leading to the upper water chamber of the radiator, plug the upper water chamber connection with a cloth or cotton yarn, fill the radiator with coolant, and then start the engine. When the coolant temperature reaches 80°C, the thermostat should be in the open state, and you will see the coolant in the radiator being pumped out through the open thermostat. The higher the engine speed, the farther the coolant is pumped. After pumping out the high-temperature coolant for a while, add more coolant to the radiator. The thermostat will close as the engine temperature decreases, and no coolant will be pumped out through the hose leading to the upper water chamber. If the engine continues to run and the coolant temperature rises above 80°C, the thermostat will open again. If the above actions are abnormal, you can remove the thermostat or replace it with a new one for testing to determine whether the original thermostat is malfunctioning. 2. Remove the thermostat from the engine, clean it, and heat it in hot water. Use a thermometer with a range of 100°C to measure the temperature. Check whether its performance is good and its operation is reliable according to the specified temperature at which the thermostat valve opens or the side valve closes.

[MacGianna]

To check if the thermostat is working properly, my experience is to mainly observe the changes in coolant temperature. Start the car when it's completely cold and keep an eye on the temperature gauge. If the temperature rises unusually slowly or never reaches the normal operating temperature around 90 degrees Celsius, it's likely that the thermostat is stuck in the open position and can't close, causing the coolant to continuously circulate and cool down, which prevents the temperature from rising. On the other hand, if the temperature shoots up extremely fast or even causes the engine to overheat, the thermostat might be stuck closed and unable to open normally to allow coolant circulation for cooling. There's also a simple method: after the engine has warmed up, gently touch the inlet and outlet hoses of the radiator. Normally, the outlet hose near the engine should be noticeably hot, while the inlet hose near the radiator should be warm at first and only get truly hot after the thermostat opens. If there's always a significant temperature difference between the two hoses or if both remain cold, the thermostat is probably faulty. Don't attempt to replace it yourself; have a mechanic confirm the issue by disassembling it.

[DelLiam]

To determine if the thermostat is functioning properly, I find the most practical method is to monitor the temperature gauge's behavior. After a cold start, if it takes over ten minutes for the needle to reach the midpoint or remains stationary, the thermostat likely isn't sealing properly, allowing coolant to circulate through the main loop. Conversely, if the needle shoots into the red zone within just two or three minutes, it's a clear sign the thermostat is stuck closed, failing to release heat. My dad's old car once suffered from this—climbing a hill caused the temperature to spike, and we later discovered the thermostat was rusted shut. The classic test is to remove it and boil it in hot water to see if it opens and closes at the specified temperature. Additionally, if the cabin heater blows cold or takes too long to warm up in winter, the thermostat could be the culprit. Though it seems insignificant, a faulty thermostat can leave you freezing in winter or risk engine overheating in summer. It's wise to also check the radiator and coolant condition while you're at it.

[Geoffrey]

To test the thermostat, I usually measure the actual water temperature changes. After a cold start, use an infrared thermometer to check the coolant outlet on the engine block and the upper hose of the radiator. If the temperature at the engine outlet reaches around 90 degrees Celsius while the upper radiator hose temperature remains significantly lower, it indicates the thermostat isn't opening or has limited opening. A more intuitive method is to remove it and boil it in water: place the thermostat in a pot with cold water and gradually heat it. Observe whether the main valve opens at the set temperature as it should. For example, if it's marked to open at 88°C, it should start moving at that temperature. If it fails to open, opens or closes prematurely, or if the valve movement is weak or stuck, it's considered faulty. Note that during testing, the bottom of the valve should be at least 25mm from the pot's base. This method is reliable, but be careful not to burn your hands!