The first thing to check is the cooling fins on the radiator. If they are badly corroded and crumble when touched, they will not be able to give up the heat to the air. The clue is the overheating stops when you run the heater fan on the higher speeds.
While it is not common for this engine, consider having your mechanic perform a chemical test at the radiator to look for a leaking cylinder head gasket. If combustion gases get into the cooling system, they will pool under the thermostat and prevent it from opening. Thermostats have to be hit with hot liquid to open. Hot air will not do it.
Tuesday, June 13th, 2017 AT 7:54 PM