and if cleaning doesen't work, you could try ordering this part to replace if you want to try before going to the dealer (make sure to use your own VIN to identify the part before ordering) :

http://bmwfans.info/parts-catalog/E7...alve#7537113_1

but of course could be something else causing the valve not to function correctly although the valve is good, do you have any other fault codes?

what this valve seems to be doing, googled tank valve :

Responding to concerns about the escape of hydrocarbons into the atmosphere we have developed a new generation of vent valves that use carefully calibrated springs to control a two way valve. This system is used on the TPV6 and the TPV8 In-Line valves. The benefits are that valve is shut when the engine is switched off so there is no trace of fuel smells in the garage. The valve will open to allow air into the tank to replace the volume of fuel that has been consumed. The valve will also open should any excessive pressure build up in the tank. This function safeguards the fuel system.