Idle Control¶
A closed-loop controller that holds the target idle by adjusting the throttle blade — with cold fast-idle, a dashpot, and anti-stall protection.
What it is¶
When you let off the throttle, the ECU takes over idle. Idle control is a PI controller (a proportional-plus-integral feedback loop) that nudges the throttle blade open or closed to hold the target idle RPM. It includes several refinements:
- Cold fast-idle — the target itself rises when cold (see Cold Idle).
- Dashpot — eases the throttle closed on release rather than slamming it, preventing a stall or dip.
- Anti-stall — if RPM sags below roughly 600 RPM, the controller adds air aggressively to catch the engine.
- A non-linear throttle/butterfly flow curve — because a throttle blade's airflow isn't proportional to its angle, the controller compensates for the curve.
Why it matters¶
Idle is where an engine is most fragile — low energy, prone to stalling, and sensitive to load (imagine switching on a big accessory). A good idle controller keeps RPM rock-steady through all of that. A poorly behaved one hunts (RPM swings up and down) or stalls.
How to tune it¶
Set a sensible target, then tune the controller's response so it holds steady without hunting. Remember that idle stability also depends on the build — a light flywheel/crank and an aggressive cam make the controller's job harder.
Watch out
Idle hunting can come from the controller or from an erratic idle VE cell (common with big cams). Stabilize fuelling at idle before blaming the controller.
Rule of thumb
If it stalls when you blip and release, the dashpot/anti-stall needs work. If it oscillates at rest, the PI response is too aggressive.
In the game¶
Idle behavior is visible on the RPM gauge whenever the throttle is closed. The cold target steps down as the engine warms.