The engine is overheating. Eventually serious damage will result. The fact that running the heater brings the temperature down suggests this is not related to an engine problem. It is related to a cooling system that is not doing its job of getting rid of normal heat. The first suspect is the radiator fan clutch. Next, if this has been getting worse slowly over time, check the cooling fins on the radiator to see if they're corroding away. Feel across the radiator to see if there's any cold spots. That would indicate some of the tubes are blocked.
Tuesday, January 8th, 2013 AT 9:06 PM