When idle control is active, this table adjusts ignition timing based on how far engine speed is from the Target RPM. Below target, it adds advance so the engine makes a little more torque and pulls itself back up. Above target, it takes timing out so torque drops and rpm settles down. You can map it in up to three dimensions.
Why use timing at all when you already have an idle valve or DBW throttle? Speed. Air changes take time to reach the cylinders, while a timing change takes effect on the very next firing. Using both, timing catches the quick dips and flares (a fan kicking on, the A/C compressor engaging, a clutch being let out), while the airflow side handles the slower, sustained corrections.
How we set it up
- Shape the table so the correction grows the further rpm strays from target.
- Keep values small near target so the idle isn't constantly hunting between advance and retard.
- Make sure your base idle timing leaves room to add advance. If idle timing is already high, there's nothing left for this table to give.
- Log rpm, target and ignition angle while switching loads on and off, and adjust from what you see.
If idle still wanders, look at the airflow side too: base closed loop, PID gains and target rpm are all part of the same idle control system.
