Hissing means refrigerant is no longer in liquid form. Get a can of virgin R134, a gauge, and recharge. Will take you all of 10 minutes.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black