Get a boost gage and a manual turbo boost valve and adjust it to 12 psi. Check the injection timing or adjust it to 12 degrees retarded (static). Not hard to do. Another thing is to check is cam timing. Chain stretch starts around 180 miles depending on how often the oil was changed. There are woodruff keys available to advance the cam to correct for chain stretch. None of this is all that hard to do but a factory service manual helps explain the process. Of course you're going to need proper tools.
I remember when I bought my first 300sd the car didn't have much power but still got fairly good mileage. After I got it running strong it didn't get the same mileage but I didn't care. It's nice to have some umph under foot.

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