FWIW=
Check and clean the boost pressure sense tube from the manifold to the pump. Also check to make sure the over boost relay/valve is not stuck or blocked. Check the pressure switch that runs the relay.

My 87 300SDL had enough carbon after 220K miles that there was no boost signal getting to the IP.. The valve was gummed up. I cleaned it out with a shot of Kroil, and after it was working, I later replaced it with a new one to prevent troubles later on.

AG

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1985 300D - 190K Miles - 26 MPG on first trip home - getting rust fixed and repainted.
1987 MB 300 SDL - 238K miles - Sold
2003 DC Ram 2500 QuadCab 6 speed, 4x4 Cummins - 82K miles - 20+ MPG
1999 VW A4 Jetta TDI 5 speed - 83K miles - 50+ MPG is easy!