I looked at a 84TD a few weeks ago. I was considering buying it and it had complete records. It had its turbo replaced with a used one by Stuart Ritter at less than 200k. It was doing the same thing yours was. I think the price for the used turbo was around $300. If Ritter puts in used turbos it must be a reasonable course of action in some cases. It probably depends on the known history of the used turbo.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13