severe 300d fuel problem

[spencoid]

I tried bio-diesel once in my 1984 300TD and have been having problems ever since. The problem may not be related to bio-diesel but I am mentioning this just in case.

I have been having severe fuel starving (I think) problems under load. Chaning the fuel filters seems to fix the problem but it re-occurs after as few as 50 miles of driving. I thought it might be crud in the tank so I removed it and completely cleaned and dried it. Still have the problem, maybe worse. the last two fuel filters did not seem to have anything in them on autopsy. I have not checked (just thought of it) to see if it might be a problem with vacuum lock of the fuel tank and maybe the ritual of changing filters releases this.

I measured the fuel pump intake vacuum acording to the manual and also measured the output pressure with conventional vacuum and pressure gauges (not the official test set). Suction and output pressures are both within range but with extreme pulsation. Is the MB gauge heavily damped to read an average value or does this pulsation indicate a bad pump? I haved a spare that I can try but no replacement gasket and am in the middle of nowhere and don't have time to get a gasket. I can make a gasket but do not know if the thickness is critical for correct operation of the pump as it is camshaft driven. Difficult to know what the thickeness should be since the gasket comes off in pieces on such an old car.

I live on a dirt road,far from town so it is very difficult to test preformance. It seems that I can drive most of the day on the highway with only intermittant power problems and that after riding on the dirt road for a while it gets worse and cause the car to eventually stop requiring a filter change again.

Does bio-diesel destroy the valves in the fuel pump? Does the pump or anything related to its preformance change under load? It seems that plenty of fuel is going through the primary filter when I race the engine and watch the filter. Can't see this while driving up a hill, of course.

Is it possible that the expansion tank system is at fault? I can not find any diagnostic information on this system in the shop manual.

Could this be an ALDA problem? Seems to severe for that. I can not climb even a slight hill when the problem occurs. Most recently it does not seem to have fully recovered from the last filter change.