When you manually shift thru the gears is the acceleration still sluggish? BTW, you can manually shift all 4 gears even though there are only 3 positions on the gear shift ..... if you move the selector back to "L" as soon as it shifts into 2nd gear, the tranny won't jump into 3rd until you select "S" again. I had to manually shift my 240d in this fashion for a few days when the white plastic valve that Dr Parts & Steve mentioned failed. If I didn't know that the valve was the culprit, I would have more than likely assumed that I had a fuel related problem. If I left the vehicle in "D" from a standstill, it would have taken an eternity to get up to speed since it would go into 4th gear at about 25mph. Also, just curious if this a problem that developed recently, or has this been the case since you've owned the vehicle???
1980 Mercedes 240D
1982 Mercedes 300SD
1985 Mercedes 300D
1993 Dodge Cummins Turbo Diesel