I just drive it. 190k mikes now and still gets 6 - 8/100km. I've regularly used LiquiMoly diesel additives in the past but for the past couple of years I've been using Archoil diesel treatments.

Clogged DPF, poor fuel, bad fuel filter, cold ambient temperatures, short trips (especially in cold temperatures), excessive idling, stop and go driving.

How did you clean the carbon? It builds up on the top of the valves on DI engines. Cleaning it properly is relatively invasive procedure.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim