I am assuming by smoke you mean a dark gray under acceleration. This is from being babied. These cars need to be driven. These engines don't have a typical diesel low red-line. These red-line at 5200 RPMs. If it's an automatic manually shift it and wind the gears up to the 4000 range. On your speedometer it should have dots indicating the max shift points. 1 dot 2 dots and three dots. Use them and don't be afraid to hold the petal to the floor. You will be amazed how much better the car will run if treated this way. 3 gallons of Bio diesel will help clean out the injectors as well.
Does anybody know, if there exists a repair manual for this model MB 240D W115? I would like to buy it and I was looking all over the Internet and I could not find it!
Adjust the play in the shut-off cable on the side of your injection pump. And check your pneumatic injection pump diaphragm.
My family have driven the w115 /compact since the late sixties.
I have restored my compact, and on the engine, only the head gasket and the top have bean changed and restored. Also the diesel pump has been adjusted.
To the point, my car or any of the ones we had blow out black or blue smoke, so if the engine is OK, a little in the beginning if the car hasn't bean driven. Because particles are always collected in the exhaust pipe. But after being driven a while, the smoke should be normal.
This is our experience of the w115 /compact throughout the years.
Good luck with your car.