How to Check if the Thermostat is Damaged?
3 Answers
Here are the methods to check if the thermostat is damaged: 1. To determine whether the car's thermostat is damaged, check when starting the car. After the engine begins working, inspect the coolant tank. If the cold water inside does not show any flow movement, it indicates the thermostat is damaged. 2. After starting the car, touch the upper and lower radiator hoses to feel the temperature. Generally, three minutes after the engine starts, the temperature of the upper and lower hoses should be the same. If they are not, the thermostat is likely damaged. 3. When starting the car, pay attention to the engine's warm-up time. If the engine takes significantly longer to warm up than before and the car's power is noticeably reduced, the thermostat is probably faulty. 4. When the car is cold, start the engine and observe after five minutes. If the coolant temperature does not reach the normal range of 85 to 105 degrees, the thermostat may be malfunctioning. 5. Monitor the temperature gauge. When it reaches 70 degrees, open the radiator's drain valve to test the water temperature. If the water temperature is very low, the thermostat is likely damaged. 6. Additionally, consider the car's mileage when deciding whether to replace the thermostat. Generally, the safe lifespan of a wax-type thermostat is 50,000 km. Beyond this mileage, it should be replaced.
I have an old Ford car whose temperature gauge kept spiking last year. When I took it to the auto repair shop, they found the thermostat was stuck, nearly ruining the engine. A simple check method is to start the engine when the car is cold, wait a few minutes for the temperature to rise, then feel the upper radiator hose (inlet hose) and lower radiator hose (outlet hose). If the upper hose is scalding hot but the lower hose remains cool, it means the thermostat isn’t opening, and coolant can’t flow to the radiator. If both hoses are equally hot or the temperature gauge rises too slowly, the thermostat might be stuck in the closed position. Don’t rush to dismantle anything—first, use a thermometer to measure the temperature difference. A significant temperature gap is a clear sign of trouble. Always pay attention to abnormal coolant temperature readings while driving, and don’t push it until the high-temperature warning light comes on. Once the engine overheats, it’s prone to deformation or oil leaks. I recommend addressing issues early to avoid costly engine repairs. If you’re unsure, let a professional mechanic diagnose it—they can often spot the problem just by pulling a hose.
As a car enthusiast who often does DIY repairs, checking the thermostat is actually not difficult to get started. First, let the car cold start until it reaches normal temperature (watch the gauge needle rise to the middle), then squat down and feel the radiator hoses near the engine: the inlet hose should be hot while the outlet hose should be slightly warm. If the temperature difference is too obvious, such as one being scalding hot and the other cold, it means the thermostat is stuck either not opening or not opening fully. Alternatively, use a temperature gun to measure: if the inlet is 50°C and the outlet is 30°C or more, the difference indicates it's faulty. Slow temperature rise during a cold start is also a sign. Remove it and soak it in hot water to test if it can open, but be careful of burns and coolant splashing. If you find any leaks or signs of corrosion, it's even more important to replace it. The parts aren't expensive but require patience. If unsure, don't force it—getting it checked at a shop is safer and saves time, preventing engine overheating or a significant increase in fuel consumption.
