What decel cut does
When you coast in gear with your foot off the pedal, the wheels are turning the engine, not the other way around. There's no reason to keep injecting fuel, so the Decel function in ESP stops the injectors until you get back on the throttle or the RPM drops toward idle. The payoff is lower fuel use, lower emissions and less of the lean-rich surging you can get from a small, unstable amount of combustion on overrun.
The whole function is really about two transitions: getting into the cut without a jolt, and coming back out of it without a stumble. Almost every setting is there to smooth one of those two moments.
Step 1: how the ECU knows you've lifted
On the Decel Detection tab, Mode picks the load signal:
- TPS (the default): decel is allowed when throttle position is below the User Demand Minimum value. Open the throttle past that and normal fueling returns straight away.
- MAP: decel is allowed when manifold pressure is below the Max MAP value, which defaults to -80 kPa (about -11.6 psi, or roughly 24 inHg of vacuum). Rising above it ends the cut.
TPS is the right choice for most cars because it responds the instant the driver moves the pedal. MAP can be handy on engines where the throttle signal isn't reliable at closed throttle, but a big cam that pulls weak vacuum on overrun can make it hard to find a clean threshold.
Step 2: when the cut is allowed
Tick Enable Decel Cut on the Decel Cut tab. That also brings up the Cut Delay Time table in the navigator. Then set the conditions that have to be met:
| Setting | What it does | Our advice |
|---|---|---|
| Arming TPS | Throttle has to go above this before decel cut can arm. It then activates once you lift back to zero demand | Around 1 to 5% is typical. Stops the cut flickering on and off around closed throttle. 0 turns the check off |
| Cut Activation Offset | RPM must be this far above the Min RPM table before the cut can start (default 400) | Leave a healthy gap so the cut doesn't start and stop repeatedly near the resume point |
| Post Start Delay | Blocks decel cut for this long after start-up (default 5) | Keeps start-up flare and the post-start idle offset from triggering a cut |
| Min Gear | No decel cut below this gear. Reverse counts as -1, and -3 ignores gear | 2 or 3 is common, so first gear crawling stays smooth |
| Min Vehicle Speed | No decel cut below this speed. 0 ignores speed | Set it above walking pace so the cut can't happen with the car stopped |
| Min Coolant Temp | No decel cut until the engine reaches this temperature | Cold engines drive badly with the cut on. To ignore it, set below your coldest start temperature |
| A/C Offset | Raises the minimum RPM by this much when the A/C output is on | Useful if the A/C load makes the engine stumble on re-entry. 0 disables it |
Step 3: smoothing the way in and out
Cut Delay Time table
This is the pause between all the conditions being met and the injectors actually shutting off. A short delay lets the car settle before the fuel goes away. At higher RPM, a longer delay stops the cut from kicking in during a quick gear change. As a guide, figures of roughly 200 to 500 ms at low RPM and 2500 to 3000 ms at high RPM are typical.
Ignition Retard
While the cut is active, timing is pulled back by this amount (default 10). The retard is ramped in over 500 ms before the fuel is cut, or over the delay time if that's shorter. Less timing means less torque, so the engine eases out of driving the car instead of snapping off, and the same applies when fuel comes back.
Fuel Enrichment
After a stretch with no fuel, the port walls and the back of the valves have dried out. The first injection after the cut would partly go into re-wetting them, leaving that cycle lean. Fuel Enrichment adds this amount to the first pulse when fueling resumes (default 25).
Min RPM table
Once cut, normal fueling comes back if RPM falls below this table's value. It can be mapped up to 3D. The ECU also watches how fast RPM is falling. On a gentle coast-down it lets the cut run to a lower RPM, but when RPM is dropping quickly, like after a free rev in neutral, it brings fuel back sooner so the engine doesn't dip below idle. O2 control uses this table too: above it, closed loop is paused at zero demand, because combustion during overrun is too unstable to trim against.
A worked example
Say the Min RPM table reads 1300 rpm in your cruise area, and Cut Activation Offset is 400. You're in third at 2500 rpm, with the engine warm. You lift. The throttle has been above Arming TPS, so the function is armed. RPM is above 1700 (1300 + 400), gear and speed are above their minimums, so the ECU starts the Cut Delay timer and ramps in the timing retard. When the delay runs out, the injectors stop. You coast down to 1300 rpm, fuel returns with an enriched first pulse, and the engine carries on idling smoothly as you pull up.
Troubleshooting decel cut
| Symptom | Usually |
|---|---|
| Bucks or surges while coasting | Cut switching on and off. Increase Arming TPS or Cut Activation Offset |
| Jolt when you lift off | Cut delay too short, or not enough ignition retard |
| Stumble or lean spike when fuel returns | Fuel Enrichment too low, or Min RPM set too close to idle |
| Stall when rolling to a stop | Min RPM too low, or A/C Offset needed |
| Decel cut never activates | One condition not met: check gear, speed, coolant, post start delay and that your TPS reads 0% closed |
If the car feels rough on light throttle at the same time, look at transient throttle too, because the two often get blamed for each other.
Decel FAQ
Should I run decel cut on a race car?
Usually yes. It saves fuel over a long race and avoids the weak, unstable combustion you get on overrun. Some drivers prefer it off for a consistent feel on corner entry, so it's a preference call.
Why does my wideband go full lean on decel?
That's normal. With no fuel injected, the sensor sees mostly air. Ignore those readings when tuning and don't let closed loop learn from them.
Is this the same on Nexus?
The idea is the same, but the settings live in NSP. See our Nexus decel article.
