It could be an idle air control valve.

The Idle Air Control (IAC) valve controls the engine idle speed and dashpot functions. The valve is located on the side of the throttle body. This valve allows the necessary amount of air, as determined by the Powertrain Control Module (PCM) and controlled by a duty cycle signal, to bypass the throttle plate in order to maintain the proper idle speed.

The check engine light doesn't have to be ON to store a fault code.

Check the vehicle for fault codes to see if there are any sensors were involved to the malfunction of the engine.

1.Local auto parts stores such as Autozone ( not in California) or Advance Auto parts will scan and retrieve any diagnostic trouble codes ( dtc ) stored in the powertrain control module for free of charge..

2. Amazon.com has the code reader for less than $20.00 dollars.

3.Re-post the dtc's in here " answer question " for further assistance.

Note: we need the dtc's to identify the sensor, there are a dozen different sensors to operate the car.

Visit this site and see all the different sensors.

http://www.freeautomechanic.com/car-engine-sensors.html

Navigate to this site and see the code reader

http://www.amazon.com/.../...imh_gw_p263_d0_i1

my 89 gmc idle rough in gearfine on the road at speed