It's worth pointing out that if there is a hole in any of the turbo hoses on
the pressurized side, the engine power will exhibit a "fold-back" behavior -
the power will drop suddenly when the turbo starts spinning and will stay
very low (just more than idle) until you let off on the throttle enough to
spin the turbo down, then everything will be fairly normal until the turbo
spins again.
There is a valve mounted to the turbo that shunts the turbo output back to
the input, allowing the turbo to freewheel. I understand it is to prevent
overpressure in the ducts if you lift throttle suddenly when the turbo is
spun up. Maybe it is staying open?
Mike