Get a mighty vac and do some troubleshooting.
Old Benz have lots of potential for vacuum leaks.

Might be cheap (hose), might be expensive (actuator).

Got to check it out to verify.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles