Hondata

The factory ECU logic for S. and L. Trims seem counter-intuitive. For example, when running the E25 calibration/+13% overall fuel trim with E25 fuel, the L. Trim is pretty close to 0%; S. Trim is about -2%. This is 100% factory and no AFM adjustments.

When I increase the fuel to E30 without changing anything else on the calibration, the L. Trim is about -3%. Shouldn't L. Trim be +3% because there's more ethanol? The weird thing is that S. Trim seems closer to 0.

To compensate for the E30 fuel, I increased overall fuel trims to 16%. The L. Trim is still at about -3%, but now S. Trim is more negative, about -2%.

Any ideas how the S. and L. Trim logic works? Do I need to give the ECU more time to learn? I've attached a cruise log with the E30 fuel with 16% overall fuel trim.

LTFT is calculated by rpm, so you can't really compare the LTFT between datalogs.

As long as the STFT is centered around 0% and the LTFT doesn't get too far from 0% it is ok.

Thanks. So if you wanted to tweak the L.trim to be closer to 0, what would you change? The overall fuel trim?

It depends on why the LTFT is not around 0%. If all the LTFT is negative or positive then the overall trim should be changed. If you have an intake which is changing the AFM signal then the AFM tables. If you have a hot air intake aka SRI then give up because you can never get the trims to be consistent. Finally on a heat soaked idle even with IAT compensation in the ECU no matter what you do expect the trims to wander around a bit at idle, which is normal.