Sounds like you have un-metered air getting into your intake manifold (air that is getting by the MAF). I would guess that the hose that leads to the IAC has a leak or the manifold gasket is bad. When you remove the MAF assy for cleaning you move the hose and plug the leak for awhile. Also, the upstream O2 sensor reports rich/lean, the downstream monitors the precat and has nothing to do with running lean.
The MAF produces a small DC voltage that is proportional to the airflow past the filament. The faster the air the higher the MAF voltage. This voltage is converted to a digital number or step. I donâ??t recall how many steps but itâ??s not too many. The steps are used along with other inputs compute air/fuel ratio and injector timing. The MAF is calibrated to the engine. All in all, the MAF gives the PCM a ballpark reading on air flow it is not a sensitive device and is not usually associated with poor performance (it either works or it doesnâ??t)
Another way the MAF could get dirty is from crankcase gasses from a bad PCV system or worn rings. You would see oil inside the plumbing if this was the case.
jeff
