What idle long term trim does
Your idle controller works in two layers. The base table supplies the duty or position the ECU expects to need, and the closed loop controller makes quick corrections to hold target rpm. If the base table is a little off in the same place every time, the controller keeps making the same correction, over and over.
Long term trim notices that pattern and remembers it. It builds a correction table over time, so the next time the engine is in that condition, the correction is already there. The closed loop side then only has to handle small, short-term changes, which makes idle steadier.
The three parts in NSP
| Item | What it does | Our advice |
|---|---|---|
| Long Term Trim (settings) | Turns learning on or off, resets what's been learned, and has an Apply to Base Table button | Reset after any big mechanical or base-table change so old learning doesn't carry over |
| Long Term Trim Gain | How quickly the learning can change the trim table. Bigger numbers learn faster, smaller numbers learn slower. Up to 3D. | Start low. Fast learning can chase itself and make idle oscillate. |
| Long Term Trim (table) | The learned correction values. You set the axis values, but the axis channels always match the Base Duty Cycle table. Up to 3D. | You can edit it while learning is running if you want to nudge a cell by hand |
How we use it
- Get the base idle table close by hand first. Long term trim is for fine-tuning, not fixing a bad base.
- Enable learning with a conservative gain and let the car idle through warm-up, with the A/C and other loads cycling on and off.
- Watch the trim table fill in. Consistent corrections in the same cells mean the base table is off there.
- Press Apply to Base Table to fold the learned trims into your base idle table, then reset the trims and repeat until they stay small.
If idle starts swinging up and down after you enable learning, lower the gain before you touch your PID gains. The learning may be fighting the closed loop controller.
See the main idle control guide for the rest of the setup, or get in touch if your idle won't settle.
