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I have a 1991 Camaro RS and the cooling fan only comes on when the AC is on. When the AC is not running, the car continues to over heat sitting still. I don't think it is the thermostat as the temp gauge also goes into the max temp zone.

If the fan comes on when the a/c is turned on then we know the cooling fan circuit is working properly. Radiator cooling fans will not come on this model till the engine coolant reaches around 235 F. When you turn the a/c on, the fan comes on when the A/C high pressure reaches 200 PSI regardless of engine temperature. What I suspect is that you don't have a good pressurized cooling system! Let me explain, your radiator is like a pressure cooker. It holds pressure in the cooling system to raise the boiling point past 212 F. Since the cooling fans don't come on until 235 F, the only way you can get it that hot is to have a pressurized cooling system! I suspect the radiator cap that may be only holding 8 or 9 lbs of pressure, instead of the 15 or 16 that is needed to allow the coolant to reach 235 F. Once it gets to 235 this allows the fans to come on. Please go to a shop that can plug a “scanner” into your car to read the actual temperature. You may be surprised that it is not getting as "hot" as you think. It may be "hot enough to boil over" and show hot on your gauge, yet not hot enough to turn the fans on. The sensor that reads your gauge is not the same sensor as the one that turns on the fan. The sensor that is controlling your gauge may be getting "hot" from steam from an un-pressurized cooling system!

 

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