i have a 1987 buick centry custom 3.8l idles high. have replaced the idle air control valve but still idles high. if you let it run a few minutes then turn it off then start it again it will idle normal.service engine soon light is on but i have no reader.
You don’t need a reader. Use a paper clip and short the A and B terminals on the diagnostic connector. Follow the guide on this web page: http://www.troublecodes.net/GM/
You don’t need a code reader.
If you look under the dash below the steering column you’'ll find the ALDL connector. Look on this connector for the A and B terminals. Using a paper clip or something simular, Jump between the A and B terminals. Turn the ignition switch to the RUN position and the Check Engine light will start to flash out codes.
The first code that will be displayed will be a code 12. And will look like *-. *= a flash and -= a pause. This code will repeat three times and means the system is under the diagnostic mode. Any codes that flash after this code are the actual diagnostic codes and will also repeat three times. So if for example there’s a code 35 it will appear as -. So get a pen and paper and jump the ALDL connector and get the codes. Then come back and list the codes you pulled.
If you engine temperature sensor is bad, the ECM might think the engine is cold and raise the idle speed accordingly.