Bob R. 6/3/2010Your coolant (anti-freeze) is low! Use only BMW Coolant in BMWs. They have a special formulation that prevents your radiator and heater from clogging up. It is about $20/gal at the dealer. If it is only slightly low, you can just add water.
You mentioned that your temp gage rises when it's hot outside. That says you are low on coolant also.
Check the level when the engine is cold. The tank is on the left of the radiator when you open the hood, and it has a mark where the level should be when the engine is cold (KALT in German).
If you ever overheat this engine, the cylinder head will warp or crack and it will cost you $4000+!
You should replace the coolant every two or three years.