What are the symptoms of a faulty car thermostat?
2 Answers
There are two main symptoms of a faulty thermostat. Here are the details: 1. If the thermostat fails to open, the temperature gauge will display in the high-temperature zone, but the radiator fan won't turn on. The engine temperature will rise, and it's prone to losing coolant. 2. If the car thermostat fails to close, the water temperature will rise slowly, especially in winter. After 10-20 minutes, the water temperature may still be in the low-temperature zone, and the idle speed will be high. Here are some methods to diagnose thermostat failure: 1. Inspection after engine start: Open the filler cap of the coolant reservoir and check if there's any coolant flow. If not, it indicates a damaged thermostat or debris stuck between the main valve. 2. You can judge by feeling the temperature of the upper and lower radiator hoses. Start the engine and touch the hoses after 3 minutes. With a functioning thermostat, the upper and lower hoses should have different temperatures—typically, the upper hose is hot while the lower one is cool. When the engine reaches about 90°C, both hoses should be hot, indicating a good thermostat. If the upper and lower hoses remain at the same temperature after starting the engine, the thermostat is faulty.
Last time my car's thermostat failed, within just ten minutes of driving, the temperature gauge shot up to the red zone with warning lights flashing nonstop—almost cooked the engine. Pulled over to find coolant boiling and smoking, so I immediately shut it off in panic. Another common symptom was the heater vents blowing cold air in winter, no warmth at all, leaving me shivering while driving. When the thermostat gets stuck, the engine temperature goes haywire, causing the heating system to fail. My fuel consumption also increased—used to be 7L per 100km, but then rose to over 8L, likely due to efficiency disruption. Worse yet, in traffic jams, the coolant temperature fluctuated wildly, making the gauge swing crazily. Only after replacing the thermostat did things stabilize. If you notice similar signs, get it fixed immediately—don’t delay, or you might end up with a dead engine.
