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Decel Fuel Cut (DFCO)

When you lift off the throttle at speed, the ECU cuts fuel entirely — saving fuel and giving clean engine braking. Hysteresis keeps it from flickering.

Demo build: this panel is locked

The DFCO DELAY control is available in the full game. See Demo vs Full Version.

What it is

DFCO — Decel Fuel Cut-Off — recognizes that a closed throttle at high RPM means you're slowing down, not asking for power. There's no reason to burn fuel: the wheels are turning the engine, not the other way around. So the ECU sets fuel to zero until you ask for power again or RPM falls toward idle.

It uses hysteresis so it doesn't rapidly toggle on and off near the threshold:

  • Engages when RPM is above idle target + 400.
  • Disengages when RPM falls below idle target + 100.

The gap between those two thresholds is the hysteresis band. Other behaviour:

  • Entry delay — fuel keeps flowing for DFCO DELAY after lift-off (default 0.6 s, adjustable 0–3 s). This is the main source of unburnt fuel for pops.
  • Predictive resume — fuel returns early if RPM is falling fast (the ECU looks 0.4 s ahead).
  • Cold inhibit — no DFCO while coolant is below 55 °C. Cutting fuel cold empties the port film, which refills slowly and causes a late re-light and stalls. The throttle box shows OVERRUN instead.
  • Re-arming — it re-arms only after you touch the pedal again, which prevents chatter.
  • Throttle position — the throttle stays at the idle-air position during DFCO, so torque returns the instant fuel does.

Why it matters

  • Fuel economy — cutting fuel on every overrun adds up.
  • Engine braking — with no fuel and a closed throttle, the engine provides clean, predictable braking on deceleration.
  • Smoothness — the hysteresis band prevents an annoying on/off surge as RPM hovers near the cut point; fuel resumes smoothly before idle so the engine doesn't stall or stumble.

How to tune it

DFCO is mostly automatic, governed by the closed-throttle condition and the RPM thresholds relative to idle. The thing to understand is the resume: fuel must come back well before idle so the engine catches cleanly as you slow to a stop.

Rule of thumb

If the engine surges or stumbles as you decelerate to a stop, the issue is usually the DFCO resume threshold or the transition back into idle control.

In the game

The status word in the throttle box tells you what DFCO is doing: DFCO… (the delay is running) → DFCO (fuel is cut) → OVERRUN (off the pedal but well above idle, for example when cold) → IDLE. The closed-loop badge shows OPEN (DFCO). The delay field is IGNITION page → REV LIMITER card → DFCO DELAY (in ms).