[William lloyd]

I am experiencing a very odd problem with the fueling on my V reg 2874cc Diesel Sptinter. On a partial throttle it will begin to knock slightly and produce black smoke . If you do not change the throttle position it builds and gets progressively worse. Under load its absolutely fine. Engine has had regular maintenance and has only done 90.000m . It has been on a diagnostic machine and is showing no fault so I presume that it is something to do with the fuel pump but would appreciate some advice from someone with some experience.