hello i'mnew to merc.i just bought a 1982 300d ,and it needs a lot of help .the vac. pressure is at 10hg, and the needle bounces around like crazy at idle.just ran some deisal purge through and changed the filters, are they suposed to be blk. motor still runs rough andhas no power at idlecan this be because of air in the fuel system ?the primer pump leaks like crazy when you try to primethe filter.

Bad vacuum pump check valve. Replace the primer pump with the updated version.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

It is too early to call it a basket case. Obviously the prior owner did little to keep it up. A lot of archives reading is in order in my opinion.

The black relates to the oil filter I hope. That is normal. It sounds like you will be chasing details for some time. The fuel if black is probably growth in the tank. Maybe even treated and showing up in the fuel filters.

Look up the boost line in the archives as obstruction is common causing no power. Many of your problems may be mostly labour intensive rather than spending money. If your idle is fairly smooth and your transmission shifts well things may not be too bad.

For your leaking prmer pump. Yes that can be an issue. If it is the older design with a white cap rather than black. Stretch a ballon over it and down on the cylinder barrel. Squeeze out the air for now. This should stop any air getting in the system from the primer pump for now until you get a new one.

Hi
In addition to the other suggestions, I'd buy at Diesel Compression Tester at Harbor Freight ($25) and do a warm (85C.) compression test. If some cylinders are low, adjust the valves and retest. Its good to know you have a solid engine before spending a lot of work and money on it.
Good Luck,
Joseph