Do you have a boost gauge installed? If not and if you've never messed with the wastegate then you're probably only running about 7-8 psi of boost, way lower than the specified 13.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo