Well... Pulled enough apart to get a good look at the wast gate flapper / turbo :

1.) Flapper is free moving with linkage removed.
2.) ~12 psi applied to Waste gate actuator causes waste gate to open.
3.) Flapper closes fully when pressure removed; cycles when applied and released.
4.) Trubo is free spinning with no noticable axial or radial play.

5) Boost is ~6.5 psi (even checked my boost gauge tubing rig into cabin by applying pressure to entire line)

Can plugged CAT (MB replaced turbo and trap oxidizer around 100k ago) cause low boost? Logically it seems like it could... Can you run the car staight-piped long enough to see if boost gets better (and wake up any late sleeping neighbors) ?

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1987 300SDL 216k
2003 F350 7.3L + Banks - 315k