1992 Buick Century

Can you check for codes, what are they? OBD1, you can use the CEL, to check for codes. 12 pin data link connector, top row, 2 pins A&B, far right hand side. Use a jumper wire between the 2 pins, turn the key on, just till the dash lites come on, no crank. The CEL should start flashing, you count the flashes to get the codes. There is information all over the web about this procedure.

As far as the high idle, you might check throttle linkage, see if it is free, also check the throttle plate, see if it is closed, at idle. On some high idle problems, I have blocked off the iac passages, to see if there is an effect on the idle, there should be a big effect. You could probably use a flat piece of wood over the mouth of the throttle-body, to block air.

How is the fuel pressure? There could be some sensor/s issues, the codes might point you in the right direction.

See below

Try cleaning out the throttle plate and inspect for a kinked accelerator cable and check the throttle position sensor-