What could be causing my BMW 3 Series diesel to smoke under acceleration?

I have a BMW 3 Series Touring diesel automatic bought from new in September 2006. I have had 50,000 miles of excellent trouble-free motoring and arrange regular servicing at the local BMW dealer. Recently I have noticed black smoke emissions from the exhaust when accelerating. Is this an indication of something serious that needs attention? Can you advise please?

Asked on 14 September 2013 by KR, Welwyn

Answered by Honest John
Might be caused by a failed turbo bearing oil seal.
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