I had the same issues on a 1990 300E 2.5 Turbo. Would operate when cold,. As soon as warmed up no turbo. Never did resolve it.

Looking on eBay for entire turbo could be an option? The older 300d turbos I've seen are cheap.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)