What closed loop idle adds
In closed loop mode, the Nexus watches engine rpm and changes the idle control motor duty to pull it toward your target rpm. The Base Output table is the starting duty, and the correction is added on top.
The base table should still be tuned so that, with no correction at all, the engine idles at the target. Closed loop is there to handle small differences, like an A/C compressor, a cold morning or a fan kicking in. It shouldn't be propping up a wrong base.
How the corrections work
- Short term trim: adds or subtracts duty right now, whenever rpm and target disagree.
- Long term trim: optional. It learns from the short term corrections over time and adjusts the base table, so the short term trim has less to do. See long term trim.
The table can be mapped in up to three dimensions, so you can add a second input if your engine needs it.
How we tune it
- Tune the open loop base first, so you know the duty that gives your target rpm across temperatures.
- Copy those values into the closed loop base.
- Switch to closed loop and log short term trim. If it's always pulling one way, correct the base in that area.
- Only then tune the PID gains.
A good base means the trim stays small and the idle stays steady.
