The Bypass valve....
The valve is there to prevent cavatation of the rotors (meaning, keep them from hitting each other and making noise), and to get better gas mileage. It allows part of the blowers boost to bypass the intake and go back into the intake plenum.
Make sure you have vacuum at the line going to the bypass valve. If not, find a vacuum source and put it on there. This should tell you if the valve works. Other way to test is a vacuum pump (hand pump). Only about 8 in. of vac should open the valve.
If this does not happen, you need a new bypass valve. Since they are now obsolete, (GO FIGURE!!!), you need to find someone that is parting it out.
Good luck,
Don