The turbo probably needs to be rebuilt. I know that DIY rebuild kits are available, but have never done it my self. I was quoted about $300 to have a local turbo shop rebuild the turbo.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine