Is the vehicle consuming oil if so do a leak down test and then maybe you have a better chance of further prognosis of your vehicle. It could be the oil ring or valves or even low compression on any one cylinder.
Mercedes Diesel White Smoke In Exhaust No Coolant Loss What Is Wrong?
Does this happen when its really cold. a lot of Diesel's will blow white smoke when its really cold when they are starting this is Diesel that hasn't burnt in the engine and has just vaporised.