Check to see if your pre-filter is backwards. They flow one way. My dim-witted self did that once and had the same running issues you are having, flooring the pedal, traveling a few feet, low speed than choking out. I had put the pre-filter in backwards. Fortunately, it was the VW & not the Benz I had done this to. The car had enough diesel to idle steadily, but not to move.

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1980 300SD (Mr. Sparkles), 1976 450SEL, 1982 300D turbo, 1989 VW Cabriolet morphing into a td cabriolet , 1997 TDI Passat, 1996 piece of Jeep,1993 F150, 1990 F250 7.3L...i think that's it, for now.