I need some clarification here. I always thought mixture control in warm idle and part throttle is in closed loop mode. ECU monitors the O2 V and ajusts the pulse width so it stays between .1 and .8V and cycles back and forth. So in that case, why is there a need for idle and part throttle LTFT? Since that uses the reading based on the O2 to begin with. Seems like that's really only needed for open loop mode. Does the fuel trim also montior WOT open loop mixture control to compensate for a bad MAF or system leak?

TIA