Setting the idle correctly on a Honda Civic (or any Honda) is very important. If the idle is not set correctly the car will either idle too low, or it will idle too high causing the idle to surge up and down.
- Warm the car up to operating temperature. This is generally determined when the radiator fan turns on when the car is stationary.
- Unplug the IACV valve on the back of the intake manifold. This step needs to be done since the IACV valve is what the ECU uses to dynamically control the idle by allowing more or less air to bypass the throttle plate. Be careful not to burn yourself!
- Set the idle to 600rpm using the idle control screw. This screw controls the static amount of air that bypasses the throttle plate.
- Turn the car off and plug in the IACV valve.
- Reset the ECU. This needs to be done because the ECU will have thrown an ECU error code.
- Restart the car. The idle should be running nice and close to 750rpm.
If you can’t adjust the throttle down to 600rpm with the IACV valve unplugged too much air is getting past the throttle plate. Check the adjustment of the throttle stop to make sure that the throttle plate is closing enough. If it is closing enough, check that the throttle plate isn’t worn letting air past. If you can’t get the idle to work properly then check that the voltage of the throttle position sensor is correct. It should read 0.5V when the throttle is closed and 4.5V at WOT. This is not easy to test. Refer to your Honda Service Manual for instructions. There are numerous other gremlins which can cause problems with your idle. Check on other Honda Forums if you’re having manic idling problems.