HouseCallAuto 8/16/2012Did you bleed the air out by opening the air bleed screw (black plastic screw) on the top of coolant tank?
Mark A.M.D 8/16/2012The system was probally not Bled correctly, Air inside the cooling system will cause an 'Air Lock' and prevent coolant from circulating and cooling. Run the engine with the cap off and coolant level on max and watch to see if any air bubbles come through, Once the engine is up to operating tempreture the thermostat will open and the fans (if electric) will engage. All of the air should be expelled by this stage. Replace the cap and test drive the vehicle, If it still overheats there is obviously another issue at play.
HouseCallAuto 8/16/2012A bad thermostat is not uncommon on BMW.