1. Make sure the engine is warm.
2. Turn off all accessories (lights, AC, radio, etc.)
3. Connect a tachometer to the engine.
4. Locate the idle air control valve (IACV). On the 1999 Civic, it is located on the throttle body.
5. Use a screwdriver to turn the idle air control valve screw until the engine idle speed is at the desired RPM. The ideal idle speed for the 1999 Civic is between 700 and 900 RPMs.
6. Once the idle speed is set, turn off the engine and disconnect the tachometer.
