The PCM is not maintaining a strict 14.7:1 AFR but within a window EPA allowed.
If it was STRICT you'd see 450 mVolts average of front 02s all the time for what your calling "closed loop" is really PCM has 7 modes of operation and as long as the EPA window is met that all the PCM has to do.
Thats clear in that the short term fuel trims window is being met then the long term values are ignored.
Decel would be in closed loop and clearly the PCM would not still be trying to maintain stoch when getting off gas pedal thus fuel trim could go as much as +23 before tripping a lean DTC.
DO not forget the stoch value can be changed in the PCM so what would happen if it was changed to like 15.2:1 ?
