Multipoint - EFI System (Part5. FUEL PRESSURE REGULATOR)

The fuel pressure regulator maintains a constant, preset pressure at the injectors. It is usually mounted on the fuel rail. After the fuel flows through the rail, it enters the pressure regulator. Extra fuel flows through an orifice in the regulator, into a return fuel line, and back to the tank.

Engine intake manifold vacuum is connected to the fuel pressure regulator. This allows the regulator to change fuel pressure with changes in engine load.

Fuel Pressure Regulator Construction

Figure below shows a cutaway view of a typical fuel pressure regulator. Study its construction. The basic parts of a fuel pressure regulator are:
FUEL INLET FITTING (allows fuel to enter pressure regulator from fuel rail).

a cutaway view of a typical fuel pressure regulator. Study its construction. The basic parts of a fuel pressure regulator are: 1. FUEL INLET FITTING (allows fuel to enter pressure regulator from fuel rail).

1. FUEL RETURN FITTING (allows excess fuel to flow out of rail and regulator and return to fuel tank).
2. CHECK VALVE (opens and closes to control fuel flow through regulator).
3. DIAPHRAGM (flexible disc that can move with changes in fuel pressure).
4. DIAPHRAGM SPRING (coil spring that pushes diaphragm toward fuel and closes check valve).
5. VALVE SEAT (attached to diaphragm, works with check valve to open and close fuel return).
6. VACUUM CHAMBER (allows engine vacuum to act on backside of vacuum diaphragm), Fig 2.18.
7. VACUUM FITTING (allows vacuum hose from in­take manifold to connect to vacuum chamber).

Vacuum chamber shown at top is sealed by diaphragm. It receives vacuum from intake manifold. When intake manifold vacuum is low (engine accelerating or under load) spring keeps bypass valve closed so more fuel is delivered to injectors. (Ford Motor Co.)

Fuel Pressure Regulator Operation

Whenever the electric fuel pump is operating, fuel flows into the regulator's pressure chamber from the fuel rail. The fuel, being under pressure, pushes on the regulator diaphragm. However, there is still not enough pressure to cause the return valve to open. Additional force must be supplied by engine vacuum.

When the engine is running, vacuum enters the vacuum chamber of the regulator and exerts a "pull" that, together with the force of the fuel in the opposite chamber, causes the diaphragm to flex and open the return valve. Excess fuel pressure is bled from the system to lean the fuel mixture. The excess fuel returns to the fuel tank. See Fig.Below.
Under rapid acceleration, the engine requires a richer mixture. The fuel pressure regulator is designed to help richen the mixture. This is what happens:
As the engine begins to accelerate, engine vacuum drops.
Since fuel pressure alone cannot keep the diaphragm flexed, it returns to its former position, closing the return valve.
This causes fuel pressure to build up higher to richen the mixture for more power.
Keep in mind that the computer and sensors are monitoring fuel mixture and other variables. They work with the pressure regulator to maintain the most efficient air-fuel ratio for the needs of the engine. Fig.Below shows a cutaway view of a fuel rail and its fuel pressure regulator. Note how the regulator acts to maintain fuel pressure in the rail and to the injectors.

Cutaway shows how vacuum affects regulator action. When engine vacuum is high (engine at low speed or idle) diaphragm flexes in direction of vacuum. This opens valve and allows fuel to bypass and return to fuel tank. At low vacuum (engine under load) diaphragm flexes down to close bypass valve and increase fuel pressure. (Honda Motor Co.)