That is one major sentence! With no punctuation it can read at least three different ways, but I can't make sense of it no matter how I read it. First of all, what led the mechanic to the coolant reservoir? Was it leaking? What led you to replace the radiator and thermostat? Apparently whoever did that was also the wrong person to work on the car. The thermostat is a very low-failure device but given the age of the car, the radiator could have corroded cooling fins. The clue is the overheating will be worse in hotter temperatures and at highway speed, and running the heater on the highest temperature and one of the higher fan speeds will bring the engine temperature down?
On many engines you have to bleed the air out of the cooling system after the thermostat or radiator are replaced. If that isn't done an air bubble can form under the thermostat. Thermostats don't open in response to hot air. They must be hit with hot liquid. If it doesn't open, the air can't bleed out to the reservoir. Look for a bleeder screw on or near the thermostat housing. If there isn't one there, look for a sensor that can be unscrewed while you fill the system.
If no other cause of the overheating can be found, have your mechanic perform a chemical test at the radiator for a leaking cylinder head gasket. That will also introduce air into the cooling system that will prevent the thermostat from opening.
Monday, September 2nd, 2013 AT 10:35 PM