Make sure your injectors aren't leaking, your fuel pressure is perfect, your O2 sensor is working (if you have one), and that you have no vacuum/exhaust leaks before you get too deep. Also note that FI, Z's with large open exhausts tend to make pops in the exhaust during decel, in my experience.
You should check all three of those sensors with an OHM meter against the factory manual. You can check them right at the ECU connector to also test the wiring circuits that lead to the sensors. I use a needle to poke into the connector, and then clip my meter onto the needles.
Idle mixture is adjustable via an air bypass port in the side of the AFM. It mainly effects idle mix only. Running mixture is not adjustable, asides from hacking the AFM tooth wheel preload. That only effects the cruising ranges. The higher load ranges are pretty much unadjustable, unless you play tricks with the readings from the sensors, have bad sensors, or modify fuel pressures.