In many other posts a tube is mentioned that is not easy to access.
If the clog in one of the two parallely running white plastic tubes in the pix below I'll try it this evening. Thanks a lot!

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE