Given the test results and dandy observations, if the overheating occurs gradually you might suspect corroded cooling fins on the radiator. That will reduce the radiator's ability to give up the heat to the air. A valuable clue is the coolant temperature will come down if you run the heater on the highest temperature and one of the higher fan speeds. The heater core is a small radiator and can give up heat pretty efficiently.
Friday, June 28th, 2013 AT 9:07 PM