Skip to content

Idle Control

A closed-loop PID controller that holds the target idle by adjusting the throttle blade — with cold fast-idle, a dashpot, and anti-stall protection.

Demo build: this panel is locked

The idle-strategy card is available in the full game. See Demo vs Full Version.

What it is

When you let off the throttle, the ECU takes over idle. Idle control is a PID controller: proportional, integral and derivative terms (the D term catches a falling RPM) that nudge the throttle blade open or closed to hold the target idle RPM. The gains are seeded automatically per build — a light flywheel gets smaller gains, to avoid hunting — and an AUTO GAINS button re-derives them. For the I4 the seeded defaults are Kp 0.00055, Ki 0.00014, Kd 0.00002, with an integral clamp of 0.15.

There are two modes:

  • Closed loop — the PID holds the target.
  • Open loop — a fixed "idle screw" airflow, BASE AIR (0–30 %; the I4's default is 6 %).

The idle target is your base target plus the cold-idle offset, floored at the fitted cam's minimum stable idle.

It includes several refinements:

  • Dashpot — eases the throttle closed after a blip rather than slamming it, preventing a stall or dip.
  • Anti-stall — if RPM falls below 70 % of the idle target (about 600 RPM at 850), the controller adds up to 15 % extra air to catch the engine. It can be switched on or off.
  • Coast-down hold — after lift-off, the throttle sits at base air with the integral frozen until RPM is within about 150 of the target.
  • The idle controller also supplies air while cranking, which is why you don't touch the throttle to start the engine.
  • A non-linear throttle 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. 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, and a target below the cam's stable floor will hunt.

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 PID response is too aggressive.

In the game

IGNITION page → right column → IDLE STRATEGY card:

  • MODE — the CLOSED LOOP / OPEN LOOP toggle
  • TARGET RPM (0–10000)
  • P / I / D
  • INT CLAMP and AUTO GAINS
  • the BASE AIR slider
  • ANTI-STALL ON / OFF
  • live TARGET, ACTUAL, IDLE AIR and STATE (which shows DFCO during a fuel cut)

Idle behaviour is also visible on the RPM gauge whenever the throttle is closed.