Diesels tend to collect soot in the internals when driven at low load because the fuel won't burn well -- the fuel delivery is short, so the temp in the cylinder drops fast. Notorious for all sorts of ills at idle that dissappear as soon as you get some rpms.

Turbos are especially bad, as the turbine gets coated with soot and loses speed. Get it good and hot, and it cleans up, the valves polish a bit, the deposits in the cylindes go away, the carbon gets flushed out of the rings so they spin again, the compression comes up, etc.

Soot collection can be collosal -- my Volvo had IP problems, turbo problems, and EGR problems when I got it, so when I fixed the IP and took a long trip at night, I blew big chunks of burning carbon out the tailpipe on downgrades. Very impressive (ruined the exhaust, though) -- I guess the heat on the upgrades set the carbon deposits (about half an inch thick in the mufflers) on fire, and when I topped a hill and the fuel dropped off, the deposits burned enough to lift off and fly. Quite a sight!

Critical linear velocity for cylinder wear it 2500 ft/sec, when the oil film will break down. Rpm and stroke determine when this speed is reached. Otherwise, bearings never get going this fast in diesels, so won't wear as long as there is sufficient oil pressure. MB does a great job of matching materials, and you can drive them hard at high rpm forever so long as they run at normal engine temps and have good oil. Oil is critical....

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!