exhaust pipe comes out white smoke

I have a 2007 BMW e90 diesel , when i start the engine from the exhaust pipe it comes out white smoke and when I revs it up after 5 mins the white smoke stops.Sometimes when I press the gas paddle, the car doesn't respond as it was normal .My mechanic had checked the injectors and found one of them fault and he had changed it but still is the same. Please can you tell me what's the problem?
Regards
Kirsten
THanks.