Check your EHA valve.

It is located on the rear (towards trunk) side of the throttle body. It is a small black box. They leak an the only way to fix is replacement. You can try the O-ring replacement, but I have never heard of that working, and it didn't for me.

picture:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=26B0N9GQI26B0N9PHZ&year=1992&make=MB&model=300-TE-001&category=C&part=E.H.A.+Valve

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E