A photo would help here I suppose.

In the engine compartment, where the top of the radiator is. Should there be a trim piece covering the gap between the radiator and the sheetmetal? I notice that when the engine is off, but the aux fan is on, air passes through the gap, bypassing the radiator. Surely bmw would have designed it so that all the air goes through the radiator?

I'll post a pic tomorrow...