06 Engine Control
Idle control
PWM valve, stepper, ETB idle and the closed-loop target.
7 min readUpdated 2026-08
Idle control is a target RPM vs CLT, a position (PWM %, stepper steps, or ETB angle) and a closed-loop trim. Open-loop first: find the duty that holds your target with the engine hot, fans off, in neutral. Then enable closed loop with a small authority.
Actuators
| Type | Wiring | Notes |
|---|---|---|
| PWM 2-wire / 3-wire valve | Low-side + 12 V from main relay | Most Bosch / Toyota PWM valves. Set frequency per valve (often 100–300 Hz). |
| Bipolar stepper | ETB 2 H-bridge | Not unipolar GM 4-coil steppers. |
| Drive-by-wire | ETB 1 angle offset | Idle is a small opening on top of closed. Dashpot on decel matters more than PID. |
| Screw only | None | Valid on a cable throttle with a well-placed stop. Closed loop cannot help you. |
Closed loop
- Entry: TPS below a threshold, RPM near target, CLT above a minimum. If you enable idle loop at 4 000 rpm on overrun you will fight the driver.
- Gains: start low. An idle PID that oscillates is almost always too much I or a target that the actuator cannot physically hit (clutch in on a light flywheel, giant cam, 20° of idle advance).
- Fan / AC: add a feed-forward adder when the fan or AC clutch output turns on, rather than waiting for RPM to sag.