Have a 2004 D22 with ZD30. Has started running very badly, often but not always no power, idles badly in that engine keeps revving as if I was blipping the throttle, but the rev counter is reading about double the revs that the engine is at. Vehicle is high mileage (400,000) and used for heavy (3 tonne) towing, but has been pretty reliable. A little white smoke at idle when cold, but otherwise ok, and although engine light goes on shortly after starting, there appears to be no water in the filter. I am mechanically minded, but not familiar with diesels or turbos. Where should I start looking? Thanks