How to check a thermostat: If already in the open position, replace. Bring water near operating temperature (175 degrees). Add thermostat. Wait for thermostat to open. If it doesn’t open, replace. Allow thermostat to cool and return to closed position. A thermostat regulates coolant flow throughout a car’s engine. It is closed when your car is cool and restricts coolant from leaving the radiator. It opens once your car reaches operating temperature helping to keep the engine cool. After you turn

How to Tell if Your Thermostat is Failing? | Engine Problem | MercedesSource.com