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.
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 roughly idle + 400.
- Disengages when RPM falls below about idle + 100.
The gap between those two thresholds is the hysteresis band.
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¶
On a trailing throttle at speed, you'll see fuelling drop to zero, then resume as RPM approaches idle.