Low idle on my 1992 gmc pickup with 4.3 v6

On the bottom right side of the throttle body toward the firewall, is the Idle air control valve. Sometimes this gets carbon build up that interferes with the stepper motor. You can unplug it and remove the valve (it unscrews with a 27mm or 25mm wrench; or the medium crescent will work). Once you have it out use a mirror to look into the bore it came out of. If it is plugged or heavy with carbon you can clean it with healthy dose of throttle body cleaner... If it is really bad you can use a bore brush to loosen up the heavy stuff and flush with generous amounts of throttle cleaner.

Check all your vacuum lines since your there. replace the old brittle stuff. Check and make sure the TBI unit is snug and not loose, look for signs of bad gasket material around the base.

Spray the TBI throat and clean the butterfly. You can use a old toothbrush to loosen up the build up and prevent you from using a whole can of cleaner.

Went you have everything reassembled, be aware that it may stumble a little on start up. It has to burn off all that throttle body cleaner.

After is it idling, you can clean the TBI throat by spraying the cleaner down into the throttle while manipulating the throttle to keep the engine running. (do not over rev the engine)

If the condition continues, there is a procedure to check the IAC. It is fairly cheap at most discount part stores. ($35 to $45)