I had a fan and radiator failure recently and had a very hard time getting the air trapped in the system after replacing the parts and the engine keep overheating.
To finally get the air out i did the following: Let the engine cool for a few hours (not warm, totally cold), removed the radiator bleed screw, remove engine coolant sensor (located under the intake manifold, #7 on this link:
http://www.realoem.com/bmw/showparts...g=15&lang=enUS ),