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 !!
Poor coolant circulation in the cooling system.
Low coolant? Cooling system airbound? Low coolant? Cooling system airbound?
Low coolant? Cooling fans not working? Water pump not circulating coolant? Radiator plugged or restricted?
Low coolant? Cooling fan not working?
It can prevent coolant flow, which can cause a catastrophic overheat.
I'm not a mechanic / technician but : - low engine coolant - engine thermostat stuck closed - water pump not circulating - engine cooling fan not working properly - restricted radiator
If coolant is up to proper level and cooling fans are working - should not overheat
Coolant doesn't go through the cooling system, and the vehicle will overheat.
I would suspect the cooling fan is not working properly. One of more of the cooling fans should run continuously when the A/C is on. If not the engine will overheat.
water pump or cooling fans or thermostat low coolant
you need to burp the cooling system
lack of coolant . no cooling fan operation, low engine oil, improper coolant mixture, thermostat stuck, debri on radiator , restriction in cooling system.