OP, DK:
It may be worth noting that the O2 feedback system that includes the frequency valve is an aftermarket lashup that was installed on grey market cars to soothe the EPA. It affects the control pressure, but can only adjust in the rich direction (by reducing control pressure). In contrast to this aftermarket device, the OEM K-Lambda system adjusts the lower chamber pressure in the fuel distributor, and can both lean and enrichen the mixture.