Claire,

You most likely have a problem with the vent system. When it gets old, and especially if you tend to overfill the tank, the Diesel foam that moves through the vent lines can soften the rubber fittings (small strips of rubber hose that connect the harder vent tubing to a small separator tank in the trunk, and then to another hard line that goes to a vent fitting behind the bumper covering that wraps around the passenger rear corner of the car). As you use fuel, air is drawn in through this system, and if the rubber tube connectors are soft and gooey they will get glued shut. As you use fuel you will create a vacuum in the tank. This more of a problem on long trips after just having filled up. The little venting vessel is behind the passsenger side plastic sheet covering the inside of the fender in the trunk.

This happened to my father-in-law and he noted the gas guage did some odd things, like have the reserve light come on when he had half a tank full of fuel, the engine shut off like he was out of fuel, and the fuel filler cap was hard to get off. When he got the cap off he heard a whooosh of air. Given that these cars have a vented tank, there should not be a major whooosh sound when the cap comes off.

The solution is to have the rubber parts of these lines replaced. It cost my father-in-law under $40 for the job at the dealership.

Good luck and I 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)