I had this problem in a car I have since sold - a 1989 300E. Mercedes dealers will install an anti-percolation kit to solve it. I don't know what the kit consists of, but this might put you on the right track.

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Mark Stetson
1995 E320 Wagon Moonstone Grey/Parchment 106K mi.
2007 E63 AMG Graphite/designo 75K mi