How do I fix a high idle problem? on 1991 Nissan 240SX

The car idled at 1100 rpms. I tried to adjust it with the idle screw, and it only dropped down to 900-800, and then the idle started fluctuating around that range as well. How can I fix this problem? I can't lower it any more with the idle adjustment screw.

Asked by for the 1991 Nissan 240SX
sounds like you may have a bad idle air control valve, i would replace it and see what happens, also check for intake leaks
And will probably want to return adjustment screw back to original position.