The 91 TPS has adjustable screw fittings. Low voltage means that the TPS is a little off center. Ideally you'd be checking this with a scantool hooked up so you can watch the voltage, but you need to adjust it back into place. Someone with the 91 FSM will stick their nose in and give you the correct voltage it should be at idle I'm sure.
Once you get the idle voltage taken care of, you'll want to check the voltage at all throttle positions and make sure you're getting a steady increase to 100% throttle. You can do that by hooking up a scantool and bringing up TPS voltage, then walking the throttle slowly while the ignition is turned on (but not in start). If you see any dead patches or sudden deceases in voltage, you need a new TPS.
1995 SSEi - Totaled on 6/6/6 by some jerk who drank at 9am. WEAR YOUR SEATBELT! It saved my life and it can save yours too.