With the heat on the heater core acts as another radiator helping cool the engine.
With the A/C on, the radiator cooling fan runs continuosly, bypassing any input from the fan temp switch. At an idle, you say the cooling fan will cycle on/off normally BUT, the engine isn't under any load at an idle and apparently the cooling fan can keep up. BUT, AGAIN, if your radiator is partially plugged, cruising down the road, the cooling fan and the "moving air rush" may not be enough to keep the engine cool. I would have the radiator checked out. With 170K miles, it could well be partially plugged. This could cause the problem you described.
Saturday, December 5th, 2020 AT 10:05 AM
(Merged)