I have a 2002 Mercedes E320 diesel automatic estate. It has done 115,000 miles and I now use mainly Shell V-Power or other brand fuel, but I have been guilty of using supermarket diesel in the past. When I need to perform 'kickdown', an enormous cloud of black smoke emits from the exhaust. My local garage told me its the turbo having a clear out and not to worry, but it happens every time I need to accelerate quickly. I use the motorway at least four times a week but only doing a 30-mile round trip each time. Should I have cause for concern?