The Warden,

Early Diesels did not have a system for recovering evaporative emissions, and therefore they do not generate over or under pressure in the tank. Diesel fuel is a bit less likely to evaporate than gasoline, and apparently they did not need such a system to pass the evaporative emissions testing of the day. Later cars, like my 1998 E300D Turbo have an evaporative emissions system, and like gas cars, you get the expected "whoosh" when the cap comes off, but I think it is air escaping, not rushing into the tank.

In any case, the intent of the venting system on the W123 and W126 vintage cars is to maintain at least atmospheric pressure in the tank, much like the old gas caps of yore when they had a vent hole in them. 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)