Q: BMW 650i 2006 started acting up: sometimes the AC doesn't blow cold air and the hot engine sign comes on.

Hi There, Overheating with the AC on is generally caused by a couple of possibilities. When the AC compressor is beginning to fail, this may incur an increased load on the motor as it works harder than usual to turn a bad compressor. This increased load causes the motor to work harder than it otherwise normally would, resulting in the motor overheating. The other possibility may be a cooling system that is not working efficiently. This may be caused by a clogged radiator, blocked or clogged radiator condenser fins, a water pump that is not pumping correctly or potentially cooling fans that are not working properly.

It sounds like you may also have a condenser fan that may not be cooling the AC condenser properly. As you may know, the job of the AC condenser fan is to cool down the AC condenser as it generates a lot of heat during the course of its primary function, which is to convert the AC gas refrigerant into a liquid form, a process that generates a significant amount of heat. As the AC condenser fan blows cool air across the condenser, this allows the condenser to remain cool, keeping the pressure down in the AC system which helps to generate cooler air. I would suggest having a professional from YourMechanic come to your location to take a look at your vehicle.