What the delay is for
Closed loop O2 control trims fuel based on what the oxygen sensor sees. But the sensor sits downstream in the exhaust, so there is always a lag between a change in the cylinder and a change in the reading. That lag matters most right after O2 control has been interrupted.
Functions like decel fuel cut and transient throttle can override closed loop for a moment. Once normal fueling returns and every O2 control condition is true again, the ECU still waits for the time in this table before it trims anything. Without that pause, it would react to leftover exhaust from the interruption, like the very lean reading from a fuel cut, and add fuel the engine doesn't need.
A real-world example
You're cruising on the highway in closed loop. The road tips downhill and you lift off. Decel cut stops injection to save fuel. At the bottom of the hill you get back on the throttle and injection resumes right away. The first exhaust through the sensor is still mostly air from the cut, so it reads lean. The delay holds trims at bay until fresh combustion gas has reached the sensor. Then O2 control starts adding or removing fuel as normal.
Setting up the table
- Axes: the table is designed for RPM on the X (left to right) axis, but both axes can use any ECU channel.
- Editing axes: select the table and press F3, then edit or add axis values. When you change an axis site, NSP rescales the table so its overall shape stays the same.
- Values: the delay time applied before closed loop resumes.
Our starting point
Exhaust gas reaches the sensor faster at high RPM because more gas is flowing, so it's reasonable for the delay to get shorter as RPM rises. Start conservative, then log lambda, the O2 correction and the decel cut status together. If trims spike right after every decel cut, lengthen the delay at that RPM. If the car runs noticeably off target for a long time after fueling returns, you can shorten it.
Troubleshooting
| Symptom | Usually |
|---|---|
| Rich stumble just after coming off decel | Delay too short, so the ECU is adding fuel for a stale lean reading |
| Closed loop seems slow to come back | Delay longer than needed, or another O2 condition isn't met yet |
| Trims swing after transient throttle events | Delay too short for that RPM range, or gains too aggressive |
Once short-term correction is behaving, long term fuel trim can learn the small, steady differences.
