IIRC the fuel
pressure regulator is not actually "controlled" by the vacuum connection. The
fuel pressure should remain constant all the time because fuel metering is
controlled by LH brain adjusting injector opening in time.
The vacuum connection is there because when the diaphragm in the regulator fails petrol leaks out. The intent of the vacuum connection is to ensure that the leaking petrol is safely delivered to the intake where it can get burn. It thus causes the mixture to be rich as the engine is receiving unmetered fuel.
Jon in AUS
The fuel pressure regulator does respond "slightly" to the manifold vacuum - low manifold vacuum (open throttle) will slightly increase the fuel pressure, making the mixture slightly richer. This works because the LH ECU goes "open loop" (not trimmed by the Oxygen Sensor) when the throttle switch (the so-called WOT switch) is made. This switch actually takes effect at perhaps 65% throttle, not "wide open". Since the LH ECU is operating from programmed maps, the increased fuel pressure from the regulator gives a richer mixture.
As I understand fuel injection systems in general (which might not apply to the 928's system) most systems are set up so that the fuel pressure is "constant" as Jon says, but it is constant across the fuel injectors
-- the pressure difference between the fuel rail and the injector nozzles should be constant. This simplifies the computer's calculations, as Jon says. If the pressure across the injectors changes, then the fuel injected for a given time will change with the pressure. But of course, the nozzles are exposed to intake manifold pressure, not atmospheric pressure; in order for the pressure across the injector to stay constant, the pressure in the fuel rail needs to change with the vacuum in the intake manifold. Therefore, the fuel pressure regulator needs a vacuum connection so that it can control the pressure in the rail relative to the intake manifold pressure.