I think the issue is where the slack "piles up" on the chain path when you turn the engine over at the crankshaft with the wrench in each direction. If the chain is pulling through the injection pump drive sprocket, and up over the cams, the slack if the chain tensioner isn't really tensioning a lot, is on the chain tensioning rail side of the crank sprocket. The relative positions of the sprockets is always "locked" by the chain because you do not introduce the opportunity for the slack to accumulate between the sprockets. Turn it the other way and the opposite happens, and it is possible the chain and sprockets won't all line up the same when you change directions again. If the chain is pretty used up, the slack can affect timing of the pump. Check how far you can turn the crank in either direction before the camshaft sprocket turns and you will get an idea of what I am talking about. You will get more than a single link of rotation, meaning slack, in the backwards direction on our older engines with worn tensioning rails and stretched chains. Which means you can loose your timing and that is never good. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)