There's a turbo boost actuator next to the turbo.. that has a pipe at one end and a metal rod at the end facing the turbo. The pipe is connected to an electronic boost valve that spins and creates a vacuum, which acts on a diaphragm inside the actuator, which pulls "up" the rod, which rotates the turbo vanes to provide more boost, when you accelerate.
As the actuator pressure drops, the rod drops and the turbo blades return to "low drag" pitch to increase the flow rate of gas (since you're accelerated now and it doesn't need to apply as much boost).
Any problem with the electronic boost valve will reduce the vacuum, hence the actuator, rod movement and turbo vane pitch won't happen, and you won't get any boost.
If the pipe is split, then you won't get a vacuum inside the actuator..
If the actuator diaphragm is worn out/split, it won't move when the vacuum is applied.. and you won't get a rod movement.
If the rod is seized, then the turbo vanes won't be rotated.
If the rod is just "loose" (if the connecting pin hole has become elongated) then you can re-adjust it).
If the turbo vanes are seized, then you won't get boost.
If the problem has just appeared out of nowhere, rather than become gradually worse, then it's most likely the boost valve or the pipe.. since the valve works quite hard (it's just an electric motor) and the pipe can perish and crack and lose vacuum.... so look at those first. Actuators and turbos are more durable than garages suggest.. so look at those last. You can adjust the rod tension if it's moving but isn't acting on the turbo because of "slop".
If those check out okay (not sure how you would test the boost valve.. if it just works it doesn't mean it's sucking. It needs to suck.) then make sure the actuator rod can move. If you detach it from the turbo, you'll be able to move it (and the turbo vanes) by hand to see that they all move easily and freely.
Ralf S.
