My understanding is that the scanner duration value is a commanded duration, not the realized duration. Please let me know if you think that is incorrect. Using that assumption, the commanded duration is modified by the ECU, based upon the upstream O2 sensor feedback. So I agree that the base amount of fuel is adjusted by the O2 sensor. But if the scanner is reporting base duration, one would not see a lot of duration change if the operating condition is fairly stable, like idle. If the O2 detects a lean condition, the fuel trim would go up, but again, I think the base injector duration would stay the same, if all other things remain the same. As far as leaks go, leaked air is unmetered, so the computer doesn't know it is being inducted. So the ECU can't adjust base duration due to air leaks. I not an expert, so I'm open to differing points of view.
The upstream O2 sensor is the one that figured out how much true air is getting in ..so unmetered air passes the MAF is then used in combustion and the 02 sensor will tell the ECU to put more fuel
