In multi-point injection systems, the fuel pressure regulator has an inlet connection from the fuel rail, and an outlet that lets fuel return to the tank.
A control diaphragm and pressure spring determines the exposed opening of the outlet, and the amount of fuel that can return. So the strength of the pressure spring determines fuel pressure in the fuel rail, and keeps it at a fixed value.
However, the pressure in the intake manifold varies considerably with changes in engine speed, and with load, so the pressure drop across the injector must also be taken into account.
For any injection duration, if fuel is held at constant pressure, then, as manifold pressure varies so does the amount of fuel delivered. That means fuel pressure must be held constant above manifold pressure.
This is done by sealing the spring housing of the pressure regulator, and letting it sense manifold pressure via a connecting hose. Then, when manifold pressure alters, so does the fuel pressure.
When manifold pressure is low, as at idling, fuel pressure is low. As manifold pressure rises, towards open throttle, so does fuel pressure.
Since the injectors are all subjected to the same pressure, they all inject an equal amount of fuel.
The quantity of fuel delivered is thus controlled very accurately by the pulse width of the injector.
Manifold pressure sensing is not required in throttle-body systems, as the injection occurs above the throttle plate, at atmospheric pressure. So fuel pressure is determined by the force of the regulator spring, acting on the diaphragm.
The injectors are sealed into the manifold by O-rings that prevent air entering, at that point. The O-rings, together with plastic caps on the injector nozzles, also act as a barrier to heat being transferred to the injector body.
For a short time after an engine is switched off engine temperature keeps rising and that can cause vapor in the fuel lines. This pump stops this with a non return valve on its outlet which maintains the pressure in the fuel line during that short time.
This pressure will decay over about 20 to 30 minutes, but it ensures effective hot-starting characteristics.
When the engine is running, the circulation of fuel ensures cool fuel is being delivered at all times, and vapor formation is prevented.
The pump control circuit normally allows the pump to operate for a few seconds only, when the ignition is switched on.
And thereafter, during cranking.
And when the engine is running above a specified minimum RPM.