96 Grand AM runnng hot when..

heater/defrost is on. Runs at a good temp. until I sit idle or I turn the heater.defrost on. Thermostat???

Probably not. Think about this. Your temps are stable in the green UNTIL you stop and or turn on your defrosters.

NO AIR FLOW = NO COOLING = TEMPS CLIMBING.


Turning on the defrosters will energize the A/C system to help "DRY OUT" the extra moisture on the windshield faster. However this also leads to higher temps due to HOT refrigerant in front of the radiator.

One ofthe main reasons that electric fans are on cars nowadays other than the engine faces sideways is, to allow a faster heatup for emissions. Once they hit a certain temp the computer commands the FAN CIRCUIT to turn on. If the Relay, Fuses, or Fan motor are bad then nothing happens. And your engine will overheat/get hot.

Check your fan fuses, relays, and fans. The motors do go dead. But I'd bet on the Fuse or relay.

Larry

Does it actualy "overheat" or is it just getting hot thats the real question here.
Typical with this model running hot and usualy in need of a cooling system flush and new thermo and coolant.

If thiers rusty lookin stuff in your resivoir you need a flush!

As Larry said, make sure you fan is working when your engine gets hot.

and Welcome to AF Derek44 !