The Idle Air Control is on the throttle body.
'89 Chevy used Throttle Body Injection, which is computer controlled. The throttle body is electronically controlled, and the idle is controlled by the computer. Check the IAC (idle air control) solenoid, and make sure it is working. It is a large cylindrical solenoid that screws into the throttle body, and should have a four wire connector on it. Also, the computer is programmed to raise the idle slightly if the A/C is on to keep the alternator charging, so be sure to check that.
Turn idle adjustment screws each bore on carb until you reach desired rpm on tach. (recommended idle speed.)
3 turns out..
14 psi
When warm, 11 psi min. at idle
retard the distributor or replace the idle air control valve and MAF sensor
does the engine have a carburetor or is it fuel injected? A carbed engine has mixture screws along with a base idle screw, a fuel injected engine the idle is computer controlled.
The throttle cable runs on the backside of the the intake manifold. There should be 2 10mm screws. Loosen them with the car idleing and let the accelerator slowly close until your at the desired idle.
At the base of the carburetor are 2 screws. One adjusts the idle speed and the other adjusts the idle mixture. Screw the idle mixture in until the engine starts missing then screw it out 1 turn. Then, adjust the idle speed screw to about 850 RPM.
you have to turn the idle screw down ?? ? ? ? ? ? ? ? ? ? there are 2 of them. one on the right and one on the left. the left one is general idle and the one on the right is for air-conditioning idle and high power output idle ( rev higher ) A vacuum line may have come off if it has started revving.
Hey Mark==I think it probably is the idle control solonlid mounted on the throtle body. Take it off and clean it. GoodluckJOE