Fuel Pressure Regulator
The relative fuel pressure in the fuel system is held constant by the pressure regulator. The design of the regulator is that spring pressure normally keeps the regulator valve closed. When the fuel pumps turns on, fuel pressure presses on the diaphragm to compress the spring and opens the valve, returning excess fuel to the tank. The higher the pressure, the more the diaphragm moves away from the return pipe, increasing the volume of the chamber, maintaining the desired pressure. Most systems operate on 2.5 bar (36 psi) gauge pressure, but some people have installed the 3.0 bar (44 psi) for greater fuel delivery per millisecond. For the 9000, standard fuel pressure regulators were as follows: pre-'86 Turbo-2.5 bar (36 psi), post-'87 Turbo-2.8 bar (40 psi), non-Turbo B202I '86 on and B234 '90 on use the 3.0 bar (44 psi).
For each millisecond of injector pulse time, the amount of fuel delivered through the injector tip depends on the size of the injector opening: that's a fixed factor. But fuel delivery also depends on the relative pressure - the difference between fuel pressure pushing the fuel out into the manifold and manifold absolute pressure pushing back. As you can imagine, the manifold pressure changes when the throttle opens. If the fuel pressure were constant for all manifold pressures, then at low engine loads, with the throttle partly closed, reduced manifold absolute pressure would increase fuel delivery. To keep that relative pressure constant as the throttle is opened and closed, the fuel pressure regulator is connected to the intake manifold by a vacuum hose. Manifold pressure acts on the diaphragm to hold the relative pressure constant.
At full throttle, manifold pressure is close to barometric, so the fuel pressure gauge reads about 2.5 bar. At idle, absolute pressure in the manifold is about 0.3 bar (0.7 bar less than barometric). Now the manifold absolute pressure pushing the pressure regulator diaphragm is only 0.3 bar instead of 1 bar. The reduced manifold pressure on the diaphragm allows it to move away from the opening, returning more fuel to the tank, and dropping the gauge fuel pressure in the distributor pipe to about 1.8 bar (2.8 absolute). The relative pressure at the injector tip is still 2.5 bar (2.8 minus 0.3 absolute). That's why fuel delivery per injector is not affected by changes in the manifold absolute pressure.
