Check the state of the rubber on the gas cap. It doesn't take long before it stops sealing well. Add to that the rubber boot that goes around the filler neck being old and no longer sealing tight to the neck, and you wind up with those exact symptoms. Both my W126 cars have had it and the fix is a new filler cap and a new filler neck seal. Both are cheap.

If that's not the problem, then there may be some sort of clogged evap or return issue, but I'd check the two seals I mentioned first.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition