Neptune idle adjustments are adjustable within the Neptune software under PARAMETERS > MAIN SETTINGS > IDLE CONTROL. Idle adjusments are achieved by adjusting the duty cycle signal to the Idle Air Control Valve. A shorter/lower duty cylce allows less air through the Idle Air Control Valve (IACV) resulting in a lower idle speed, while a longer/higher duty cycle allows more air through the IACV resulting in a higher idle speed. Sliding the IACV Duty Cycle Adjustment to the left achieves a lower idle speed, to the right is a higher idle speed:
Once you've set the IACV Duty Cycle Adjustment to where the engine idles smoothly after full warm up, set the Target Idle to this RPM. This will tell the ECU where to attempt to idle. If your idle becomes bouncy or erratic, try adjusting the IACV duty cycle left or right or your target idle higher or lower until the desired idle quality is reached. Note that when using cams with excessive overlap you may need to increase your idle speeds to achieve a decent idle. Also, if your idle air/fuel ratio is way off it may be nearly impossible to attain a smooth idle. We recommend idle air/fuel ratios of around 15:1 for the best idle quality.
On setups not using an IACV (ie: individual throttle bodies), select the box "Disable IACV Error". This will prevent the check engine light coming on for not having an active IACV circuit/no Idle Air Control Valve.
If you find that upon cold engine startups that your idle speed is too high or too low until the engine warms up, you may need to also adjust your Startup Idle settings. Startup idle gives you the ability to adjust the target idle for engine coolant temperature during the warmup process. To help the vehicle warm up quickly and properly a higher target idle is specified. The top box is the engine coolant temperature in either celcius or fahrenheit depending on your preference setting and the bottom box is the target idle setting for that temperature. The ECU interpolates between the temperature setting below and above the current temperature and picks an RPM in between the target settings for both. It is best to set the target settings for normal operating temperature to match the target idle rpm above.