On my old 220D the tank was vented to a hole in the right rear fender and the little hose got kind of gummy after a while and plugged. When I sold the car to my father in law he started having real problems and the car would stop running. Being an old geezer with lots of experience with old cars before the days of emissions controls (actually he is 85 now, and I think he has driven several hundred cars in his time) he suspected fuel starvation, check the cap for the vent hole and found none. He also noticed the fuel guage stopped working but I do not remember how it failed (either the reserve light came on or the thing showed full, regardless of how much was in the tank). Anyway, he asked me about it and I looked in my manual, and sure enough there is a little tank in the rear right corner of the trunk and it has two lines coming from the tank and one that goes to the right rear corner of the car behind the bumper. He had the line to the fender removed and replaced by a local mechanic and the problem stopped. The same set up existed on my 1975 240D.

My daughter has my 1982 240D at school and if I see her this weekend I will check it out. I do not know if the system was changed for the W123 cars, but I do know the fuel cap comes off pretty easily, and it is brand new (the old one suffered a mechanical failure of the little tangs that engage the filler neck closure lip/thread).

Hope this helps. Jim

__________________
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)