Fuel Pressure Regulator

Fuel Pressure Regulator
The fuel pump delivers fuel to the injection system under pressure, and a fuel pressure regulator is used to maintain a constant system pressure. Excess fuel is recirculated via a return line back to the fuel reservoir near the main fuel pump. The type of fuel pressure regulator used depends on the type of fuel injection system.

The three different types of fuel pressure regulator:
- CIS Pressure Relief Valve
- CIS-E Diaphragm Pressure Regulator
- Digifant Fuel Pressure Regulator

CIS Pressure Relief Valve
CIS system pressure is determined by the pressure relief valve mounted in the fuel distributor. See Fig. 4-17. System pressure can be adjusted by adding or subtracting shims.

Fig. 4-17:

Fig. 4-17. Exploded view of CIS pressure relief valve showing location of adjusting shims (1) 0-rings and sealing washer (arrows) should always be replaced.

CIS-E and CIS-E Motronic Diaphragm Pressure Regulator
The diaphragm pressure regulator, shown in Fig. 4-18, is mounted on the side of the fuel distributor. It controls system pressure, and acts as a one-way check valve to maintain residual pressure in the system after the engine is turned OFF. The diaphragm pressure regulator is not adjustable and should be replaced if system pressure is not within the specifications.

Fig.4-18:

Fig.4-18. CIS-E and CIS-E Motronic diaphragm pressure regulator (A)

Digifant Fuel Pressure Regulator
The Digifant fuel pressure regulator shown in Fig. 4-19 is a diaphragm-type, similar to that used on CIS-E systems. The most significant differences are that Digifant operates at lower system pressure, and that a vacuum line connection to the intake manifold varies system pressure according to changes in engine load (intake manifold vacuum).

Fig. 4-19:

Fig. 4-19. Schematic view of Digifant pressure regulator. Vacuum hose connection to engine intake manifold adjusts fuel pressure at high engine loads.

The pressure regulator is mounted to the end of the fuel rail which supplies the fuel injectors. It is not adjustable and should be replaced if system pressure is not within specifications.