Hi I’m having problems with a4 2.0 tdi, when starting from cold it fills the road with grey/white smoke, runs lumpy and stalls if you try to pull away, when warm it has no low down power until the revs hit about 2000rpm when it will take off and fill the road with black smoke, it’s been in the garage for 5 weeks they fitted a mass air flow sensor, map sensor, turbo, turbo pipes, injectors, they returned it yesterday and said there was nothing more they can do, has anybody else had these problems and what was the cause, any help would be grateful received