Any chance the vac mod was recently removed/replaced? It is very easy to incorrectly install the vac mod seal. The incorrect installation will result in a difficult to find leak. (ask me how I know.....)

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles