I doubt that you have a turbo problem.
There are a couple possibilities. The most common is that the turbo boost signal isn't getting to the aneroid on the top rear of the injection pump. Without that signal the pump will not give the extra fuel necessary to use the extra air coming into the motor under boost. This is easy to test with a vacuum/pressure gauge. Tee into the line that carries this signal further to the blue vacuum amplifier on left fender. This line comes from the hollow screw that attaches the line to the aneroid. Its usually easiest right at the hollow bolt. We use a section of the hard plastic door lock vacuum line to allow us to drive the car and monitor the boost gauge inside the car (be safe here please - use a second person to watch the gauge). You should have around 10psi boost at around 2500rpms full throttle. The waste gate will prevent the boost going higher.
If you don't have this boost check the boost at where it comes from the intake (this also could be done first but the answer lies in finding the restriction to the signal and verifying the true boost). The common restrictions are: the overboost protection switch-over valve and the bolt where it comes from the manifold.
If you decide that you actually don't have any boost you must verify the position of the pop-off valve. The late (85) 617 and early 603 motors used pop-off valves in the intake to prevent the turbo from loosing velocity when the throttle is let up. The concept here is that when the engine speed slows drastically the turbo speed has to also due to the pressure building behind the intake valves. This pressure slows the turbo drastically. So upon vacuum signal the pop-off valve opens venting this pressure back to the turbo intake allowing the turbo to pump in a circle instead of into the motor. As soon as the throttle is depressed the vacuum signal goes away and the engine goes back to power with the turbo at close to full speed (solves turbo lag problems).
If this stays open the turbo does nothing. I would check the vacuum to the pop-off valve or just disconnect it for testing. For vacuum to be the problem would require vacuum to be present all the time.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 04-09-2000).]