Do you have the Garret or the KKK turbo? If you have the Garret, there is a rubber hose that runs up to the wastegate diaphragm. It commonly cracks, splits, or fails. Try replacing it first with a piece of vacuum hose and see if that takes care of the problem.

If you have a failed diaphragm, you need a new one.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]