So they install a bypass valve that's vacuum operated. The valve is connected between the intake side of the turbine blade and the output (pressure) side; when the throttle closes, the vacuum in the intake manifold spikes and opens the valve and dumps the pressure back to atmosphere (that's the "PFSHEW!" noise you hear when you dump throttle with the windows down), which equalizes the pressure on both sides of the turbine wheel and avoids the shock effect.
The classic failure symptoms include engine stalling on rapid deceleration; idle dipping below normal (almost stalling) when coming to a stop.
Some question the installation of my valve (they say it's installed backwards), but I'm sure that mine is installed correctly. I think the question arises because of a drawing on the Townsend import site. I can't quite decode that drawing - maybe it's for a 900. Mine is installed correctly for the 91 9000, though. Then piston moves back toward the end where the vacuum line attaches. That means you don't want turbo pressure pressing on the piston at the opposite end (because that would force the valve open), so that connection goes to the intake (low pressure) side. The 90 degree connection goes to the metal inlet pipe, which carries turbo pressure. Turbo pressure will then be applied to the "back" side of the piston, meaning that the piston will be forced closed rather than open, until vacuum is applied to the vacuum line.
Click the thumbnail pictures for a larger view. USE THE BROWSER'S BACK ARROW TO RETURN.To check the valve, disconnect the control vacuum hose from the "Y" connector and suck on it (it's the orange hose in the photo at right); if there is resistance, the valve is probably OK (at least the diaphragm is intact). You also might be able to put your hand on the valve and feel it snap closed when you release suction quickly. You can also listen for the characteristic noise on rapid decel with the windows down. Photo at left is a view of the valve from under the car.
The valve's piston is held closed against its seat by a spring. The piston is integrated into a rubber diaphragm. The diaphragm seals on the outer lip of the central chamber. Vacuum pulls the diaphragm back toward the vacuum connection end, and this lifts the piston off its seat and allows a path from the inlet side to the boost pressure side, which releases the boost pressure (it goes back out through the air mass meter and air cleaner). Below are pictures of the valve guts.
Hey, be advised: The diaphragm on my bad valve was intact! Presumably, all I needed to do was find a leak where the two halves of the body join. I went ahead and tore it apart though, assuming that the diaphragm was bad.
Valve replacement is very easy. Working from above, loosen the clamp where the valve goes into the rubber intake duct, and then loosen the clamp where the connecting rubber hose connects to the metal pressure pipe. Now remove the hose from the pressure pipe, then use the hose to lever the valve out of the intake duct. Disconnect the vacuum line. Now you can replace the valve - be sure to orient it the same way as the old one was. Lever the new valve back into the intake pipe, reconnect to the pressure pipe, tighten the clamps and reconnect the vacuum line.