Engine overheats when heat is turned on and cools when heater is turned off heater has hot air when on no coolant loss mechanic says cooling system is finewhy does it overheat?

Either the radiator is partially plugged and t he system has been designed marginally ( they do that to save money you know ! ) so that when the heater is turned on most of the water gets routed through the heater core, bypassing the radiator and causing overheating, or the system is just designed marginally and as above. You can put your hand on the radiator grille and feel it from the front of the car ( using all due caution, of course ) and see if there are any sudden temperature changes from hot to cold. A cold spot indicates a plugged section. The only solution to this is to replace the radiator. And/or you can also put a restriction in the line feeding the heater core, so that more water will flow through the radiator. If your car has worked well in the past with the heater turned on, then I would suspect a partially plugged radiator. I've changed these myself, and it's not hard. Make sure you don't have any other systems bonded to the radiator for cooling, like Power Steering, a/c , etc. Good Luck !!