Come on people... Pete said make sure the thermostat is in place so that the coolant isnt runing throught to fast... Lets make it easier.. MAKE SURE YOU HAVE A THERMOSTAT IN PLACE UNDER THE THERMOSTAT HOUSING....People take them out so that their engine runs cooler.

 

Right, but the only flaw in that logic with a completely computer controlled vehicle is that it would REALLY struggle to reach closed loop status, causing a slump in fuel ecomony and causing a new array of problems. Even if the guy checks and has no thermostat, he's experiencing overheating problems, not overcooling. With the A/C on, you have a great amount of heat being dissapated by the condenser. The higher the heat, the higher the pressure and vice versa. I think this may be a simple fix, whether it may be a sticky thermostat, a worn radiator cap, or a partial cooling system fill. Always start with the easiest solutions.