You probably want to post this question over on the diesel forum. I don't know about the 87, but the 85 and olders had a boost sensor controlled via a plastic line that could get plugged lowering boost and effecting performance. You may have a similar problem. I don't notice a big change in 85td performance from sea level to mile high unlike the NA.

Are you saying that you have seen a decline in performance when going from sea level to 7000' or if there has been a degredation in performance while at 7000'?

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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