Assuming the car runs for extended periods without overheating as long the air conditioning is off, it is unlikely that you have a coolant circulation problem. That leaves poor heat to air transfer as the problem. The most likely cause of that would be a weak fan clutch. Use a popsicle stick to put drag on the mechanical fan from the back side (Yes, I mean letting the moving blades hit it). If you can easily stop it, then the fan clutch is kaputt and you need to replace it. If you can't stop it easily, then look for clogging in the radiator fins. I once spent a half an hour with a pressure washer blowing cottontree fuzz out of a woman's radiator. It fixed her problem. Other than that, it could be internal clogging from corrosion inside the radiator.
BTW, if you have to remove the fan clutch, you need to use an inch and a quarter open end wrench turning it in a reverse direction. Don't make the mistake of trying to unloosen the 10mm cap screws on the water pump pulley.
Dr. Loot's Assistant
Saturday, August 1st, 2009 AT 9:33 AM