Your are right about the fuel pump having two modes of operation - if you count one mode as being the initial no engine RPM moment that you turn the key from "OFF" to "ON" and before you select "START". The Bosch fuel injection computers all have a brief 3 - 4 second pressurization cycle to compensate for any internal leak down issues while your car is parked.
When a Porsche is not running the system fuel pressure should be statically held between all of the shut off fuel injectors and a screw-in check valve on the output side of the fuel pump. Of course once the engine starts and the key returns to "RUN" (RPM sensed) the fuel pump now has a steady power source.
When my car has not been driven for over a week or so - I usually turn the ignition key from "OFF" to "ON" several times to re-pressurize the fuel rails before trying to start the engine. I know that my fuel pump check valve leaks down a bit. I just haven't changed it out yet.
If the Cold Start Injector is not leaking, make sure your O2 sensor is good. The O2 sensor should be generating a continuous (1 second duration) square wave output that swings from approximately 0 to 1 volt. Each side of the square wave tells the Fuel Injection Computer that the exhaust is lean (0 volts) or rich (1 Volt) then tweaks (slightly increases and decrease) the pulse width of the primary 8 injectors. This ongoing automated air/fuel mixture adjustment always swings back and forth while trying to maintain the ideal 14:1 ratio.
Michael
