thanks for the suggestions guys! I will get a ahold of a pressure tester & a mirror and see if I can spot the leak. I am hoping that it isn't the head gasket.

BTW, does anyone know the symptoms of a heater core leak on these vehicles? Does the coolant actually leak inside the cabin, or is it somehow channeled outside the cabin?

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1987 560SL - 95K
1996 E300D - 183K
1997 E300D - 162K
1982 240D - 160K