Why cut fuel on decel?
When you lift off at speed, the wheels are turning the engine rather than the other way round. It doesn't need fuel to keep spinning, so injecting any is waste. Shutting the injectors off during that coast saves fuel, cuts emissions and gives stronger engine braking. The tricky part isn't the cut itself. It's deciding exactly when to allow it and making the return to fueling smooth.
How the ECU knows you're decelerating
The Mode setting picks the load signal used to decide that you're off the pedal:
- TPS: decel is detected when throttle position drops below the User Demand Minimum value. Open the throttle past it and normal fueling returns. This is what we use on most cars.
- MAP: decel is detected when manifold pressure falls below Max MAP (default -80 kPa, about -11.6 psi). Rise above it and fuel comes back. Useful where TPS isn't a reliable signal of demand.
The decel cut settings
Tick Enable Decel Cut first. That also adds the Cut Delay Time table to the tree.
| Setting | What it does | Our advice |
|---|---|---|
| Ignition Retard | Pulls timing while the cut is active (default 10). The retard is blended in over 500 ms before fuel goes away, or over the delay time if that's shorter | Keeps the change in torque gentle both going in and coming out. Leave the default until you have a reason to move it |
| Fuel Enrichment | Adds extra to the first fuel pulse when injection resumes | The ports and valves dry out while fuel is off. Without this, the first few cycles run lean and you feel a stumble |
| Cut Activation Offset | RPM must be this far above the Min RPM table before a cut can start | This is your hysteresis. Too small and the cut chatters on and off near Min RPM |
| Post Start Delay | Blocks decel cut for a set time after start-up | Stops the start-up flare and post start idle offset from triggering a cut |
| A/C Offset | Raises the decel Min RPM while the A/C output is on | Set to 0 if you don't want A/C to change anything |
| Min Gear | No cut below this gear. Reverse counts as -1, and -3 switches the check off | 2 or 3 is typical. First gear decel cut feels jerky |
| Min Vehicle Speed | No cut below this speed. 0 disables the check | Stops a cut happening when the car is barely moving or stopped |
| Min Coolant Temp | No cut below this coolant temperature | A cold engine runs poorly after a cut. To disable, set it below any cold start temperature you'll see |
| Arming TPS | Throttle must first go past this before the cut can arm, then it triggers at zero demand. 0 disables it | Around 1 to 5% is typical. Prevents the cut flickering when TPS sits right on the zero demand edge |
| Clutch Switch Override | No cut while the clutch switch is active | Great on manual cars. Clutch in usually means you're about to idle or shift |
Worked example
Say your Min RPM table is 1,500 RPM, your Cut Activation Offset is 300 RPM and Min Gear is 2. You're cruising in 4th at 2,800 RPM, then lift completely:
- TPS drops below User Demand Minimum, so decel is detected. Because you'd pressed the pedal past the Arming TPS value earlier, the cut is armed.
- RPM is well above 1,800 (1,500 plus 300), you're in a gear above 2, and the engine is warm, so the cut is allowed.
- Timing is retarded smoothly and then the injectors go quiet.
- RPM falls toward 1,500. When it reaches the Min RPM value, fuel comes back with the first pulse enriched and timing restored.
The 300 RPM gap is what stops it from cutting again immediately if RPM wobbles around 1,500.
Idle control has its own guard, Decel Fuel Cut Restore, that keeps decel cut away from the idle target. If the car stalls or dips as you roll to a stop, check that the Min RPM table and that guard leave plenty of room above idle. Our idle control article covers it.
Troubleshooting
| Symptom | Usually |
|---|---|
| Surging on and off while coasting | Cut Activation Offset too small or Arming TPS at 0 |
| Stumble or bog when fuel returns | Not enough Fuel Enrichment, or too much ignition retard coming back |
| Jerky in first or second | Min Gear too low |
| Cuts right after starting | Post Start Delay too short |
| Never cuts | Mode threshold wrong (TPS or MAP), coolant or speed lockout, or clutch switch reading active |
FAQ
Should I use decel cut on a race car?
Often yes, for fuel and heat, but some drivers prefer the steadier engine braking with fuel on. Try it and ask the driver.
TPS or MAP mode?
TPS for most setups, since it reacts instantly to the driver. MAP can make sense when the TPS isn't a dependable signal of driver demand. Either way, confirm in a log that decel is detected when you expect.
