I have a '81 240D. A month or so ago I noticed occasional loss of power on acceleration (considering what's normal acceleration it took a while to be sure).There was also a lot of "diesel knock" that increased in the couple hundred miles before I finally looked at the filter until it was constant during acceleration and settled down when I reached a steady speed of 65 or so.There were some black flecks in the primary filter so I replaced both filters. The knock went away and power returned to normal-problem solved. But only for a few hundred miles ( typically, my odometer doesn't work). I changed the filters again ( More flecks than before) . Problem solved for a few hundred more miles. Still more flecks. I removed the tank and had it professionally cleaned and replaced the primer pump. I replaced all the rubber fuel lines and the filters and same thing. The primary filter only fills about half full of fuel so I wonder if enough fuel is reaching the injector pump. Any advice would be appreciated.

 

I guess my post was a bit rambling and unclear but I have removed and had the fuel tank cleaned at a radiator shop and replaced the rubber connecting lines and filters. It had a lot of black particulate matter in it that the guy at the shop said was rust.I replaced the primer pump at the same time because it was leaking fuel when pumped. It didn't leak when the engine was running and the plunger was screwed in. I replaced it with the type of pump that doesn't need to be screwed and unscrewed . I don't see how this could affect it but I mention it in the interest of providing as much info as possible. The fuel reaching the primary filter seems to be clean so I wonder what else might be the cause.

 

If your fuel filters are clean then I would start looking for a leak check everything make sure you have tight, solid connections everywhere including your fuel return lines. Since you recently had the tank out look at all of the line connections there as well. If there is a weak connection it will pull air in and cause loss of power and fuel knock a lot like a plugged fuel filter.