wont go over 3000 rpm

i have a 90 pontiac grand prix with the 3.1 turbo in it and i was having a problem with it going over 3000 rpm in drive i disconnected the hose off the turbo and checked the turbines and they spun freely with the hose off the car drives like normal but when i connect it again it wont go over 3000 any suggestions would surely be grateful