How to Determine if the Thermostat is Damaged?
4 Answers
Methods to determine if the thermostat is damaged: 1. After starting the engine, check by opening the filler cap of the coolant tank to see if there is any water flow inside. If there is no flow, it indicates that the thermostat is damaged or there is debris stuck between the main valve switches; 2. Judge by the temperature of the upper and lower hoses. Start the engine and after 3 minutes, feel the upper and lower hoses. If their temperatures remain the same after starting the engine, the thermostat is faulty; 3. Use an infrared thermometer to inspect the thermostat. Aim the infrared thermometer at the thermostat housing to test the temperature changes at the inlet and outlet. When the engine is started, the inlet temperature should rise. Once the water temperature gauge reaches 70 degrees, test the outlet temperature. If there is no change in temperature, it means the thermostat is damaged and needs replacement.
The most common issue I encounter when repairing cars is a stuck thermostat. Simply put, check the upper and lower radiator hoses: if the upper hose is scalding hot while the lower one remains ice-cold after ten minutes of cold start, the thermostat likely isn't opening. If the temperature gauge shoots straight to the red zone with the cooling fan spinning wildly, shut off the engine immediately for inspection. Monitoring coolant consumption helps too – if the radiator frequently runs low without visible leaks, the thermostat gasket might be deteriorating. Last week, I fixed an old Camry with this exact problem; disassembly revealed rust-jammed valve movement, and replacing it instantly normalized coolant temperature. Remember: never push through overheating – you'll regret it when facing a seized engine requiring major overhaul.
Last time I drove a long distance, I noticed the temperature gauge kept hovering at the low end and the heater wasn't warm enough, so I suspected the thermostat was faulty. Here's a little trick: After a cold start in the morning, pay attention to how quickly the temperature rises. Normally, the needle should reach the 90-degree mark in about five minutes. If it takes seven or eight kilometers of driving for the temperature to slowly rise, there's an 80% chance the thermostat is stuck in the open position. Another down-to-earth method is to use your phone to record the radiator fan's behavior. If the fan frequently turns on within half an hour of a cold start, it indicates the cooling system isn't circulating properly. I later bought an 80-yuan thermostat online and replaced it, and the heater immediately started blowing scalding hot air.
I usually use an infrared thermometer to check: measure the temperature of the upper and lower radiator hoses before a cold start, then measure again after the engine runs for ten minutes. If the thermostat is working properly, the upper hose should be about 30°C higher than the lower hose; if the temperature difference is less than 10°C, it indicates the valve is stuck. A typical symptom of abnormal coolant temperature fluctuations is when the temperature suddenly rises at a red light and drops again when driving, which is mostly due to the thermostat not opening and closing flexibly. I remember last time when checking a car leaking coolant, I found the leakage was caused by a deformed thermostat base.
