Steve, that makes the most sense. I live in Phoenix so heat not used a whole bunch but I was thinking about the heating core yesterday and ran the heat for a bit in the morning with the thought that it may indeed be the problem. I did hear fluid "gurgling" behind the dash on the passenger side as the heat was on and don't recall hearing that on other Mercedes I have had. Thanks for the insight.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6