A miscalibrated idle can cause stalling, a rough idle, and added stress to engine components. To adjust your Honda Civic's mixture sensor, you must turn the adjustment screw on the idle speed sensor. There is no way to manually adjust the mixture sensor without altering the idle speed sensor. A handheld tachometer should be used for an accurate adjustment. The tachometer on the dash can be used if a handheld unit is not available, but the adjustment won't be as precise.
Connect a handheld tachometer to the engine. Clip the positive lead to the positive terminal of the ignition coil, which is next to the distributor. Clip the negative lead on any unpainted metal surface. Turn the tachometer to the on position for a real-time reading of the engine's rpm.
Turn the engine on and allow it to idle for several minutes, for the Civic to warm up. Wait until the cooling fan comes on, signaling that the engine is completely warmed up. Turn off all accessories, including lights, climate control and radio.
Check the idle speed on the tachometer. Compare it to the recommended idle speed in your Civic's owner manual, which may be anywhere from 550 to 750 rpm. The idle speed specifications will vary from one year to the next, and factors such as the type of transmission will also affect the target rpm.
Turn the idle adjustment screw using a flat-head screwdriver. The screw is on the side of the idle air control valve, directly behind the throttle body. Continue adjusting the screw until the tachometer shows the ideal, specified engine speed.
Wait several minutes, then check the idle again. If it has fluctuated, make adjustments to the screw to bring it back into specification. Turn off the engine and disconnect the tachometer.
