Problem: It’s very easy to spot a bad fan coupling. The engine temperature will rise when standing in traffic. On my 735i it went to about the 3/4 mark. Apearantly the electrical fan on the aircon condenser will then be switched on though the temperature switch on the radiator.
With the engine at operating temperature it was very easy to block the fan with a thin unfolded news paper. After switching off the engine the fan could be made to turn over 10 times by giving it a good swing.