My turbo doesn't turn as easily as I would think.
But that wasn't the problem for my down level of boost.

I added an aftermarket manual turbo boost controller ($20 ebay) and now I can dial in way too much boost if I want to. I don't, no point to it. I do have it set to about 0.9 atmosphere boost. I have also added boost gauge in the car to watch things.

ZOOM power is back on the highway. Couldn't get over 120kph before, now I'm scared to push it harder than 160kph just because of the age of the car, but it will go there - more pedal and more pull remain unused.

Problem is/was a soft wastegate spring, pretty sure. The boost controller side steps the issue, and nothing is easier than splicing a piece of plumbing into the wastegate hose that is external to the turbo. 10 minute job.

You could try pinching off your wastegate hose (or just pulling it off) and go for a short drive. If that addresses the problem do what I did. You'll know by 3000RPM if this is making a difference. Careful, a good working turbo can produce 3 bar with the hose off. Waaay too much boost for the car.

If that doesn't address the problem, maybe your wastegate is stuck open. Check the linkage for binding, travel. You can use compressed air (regulated to 20 psi or so!) into that wastegate hose to move the wastegate you should hear it smack shut when you take the air away. Even so, the flapper inside might still be shutting on something instead of making a seal. Any air going through there is less for the turbine (compress side)... less boost.

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Cheers!
Scott McPhee

1987 300D