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Closed-Loop Fuel Trim

Feedback from the oxygen sensor that nudges fuelling toward the lambda target automatically — within limits, and only when conditions are right.

What it is

The fuel tables are an open-loop best guess. Closed-loop fuel trim adds feedback: the ECU reads measured lambda from the O2 sensor, compares it to the target, and adjusts a Short-Term Fuel Trim (STFT) to close the gap. It's an integral controller on lambda error — the longer the mixture sits off-target, the more trim it applies, up to a limit of about ±25%.

Why it matters

Closed-loop trim corrects for the small errors no table can perfectly capture — minor VE inaccuracies, day-to-day conditions, component wear. It keeps a warm, steady engine right on its lambda target without constant hand-tuning. But it deliberately steps aside when it shouldn't be trusted:

  • At wide-open throttle — WOT runs rich for safety and power; feedback toward a stoich-ish reading isn't wanted there.
  • When too cold — enrichment is intentionally richening the mixture, so feedback would fight it.
  • When fuel pressure is too low — the ECU won't chase a mixture error caused by failing hardware.

How to tune it

Closed-loop is a fine-tuner, not a crutch. Get the base VE and lambda tables close first; let trim handle the last few percent.

Rule of thumb

Watch the STFT value as a diagnostic. If it's sitting near its ±25% limit to hold target, the base table is off by that much in that cell — correct the table rather than leaning on trim.

Watch out

A large standing trim hides a real tuning error. Don't celebrate on-target lambda if the controller is working at its limit to get there.

In the game

The live STFT readout is on the Fuel page. It should hover near 0% on a well-tuned warm engine and read zero whenever closed-loop is disabled (WOT, cold, or low fuel pressure).