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System Pressure and Residual Pressure

System pressure is the pressure value which is maintained in the system by the pressure regulator. When fuel pressure from the pump exceeds the desired system pressure, the regulator opens and routes fuel back to the fuel reservoir surrounding the fuel pump. System pressure is not adjustable and the pressure regulator should be replaced if it is faulty.

To avoid fuel vaporization and hard starting when the engine is hot, the system is designed so that fuel pressure is retained for a time after the engine has stopped running. This residual pressure is maintained by check valves in the pressure regulator and in the fuel pump. Neither the pressure regulator check valve nor the fuel pump check valve are serviceable as individual parts.

To test fuel pressure:
1. Loosen and remove the service port sealing screw shown in Fig. 7-16, and connect a fuel gauge.


Fuel will be expelled. Use a clean rag to soak up fuel. Do not disconnect any wires that could cause electrical sparks. Do not smoke or work near heaters or other fire hazards. Keep a fire extinguisher handy.


The fuel pressure gauge must have a range of at least 0 to 3.5 bar (0 to 50 psi) and must be securely connected to prevent it coming loose under pressure.

2. Start the engine and allow it to idle. Fuel pressure should be approximately 2.5 bar (36 psi).
3. Disconnect the vacuum hose from the top of the fuel pressure regulator. Fuel pressure should increase to approximately 3.0 bar (44 psi).
4. Turn the engine off. After ten minutes, residual pressure should be at least 2 bar (29 psi).

If the system pressure is too high, the pressure regulator is faulty and should be replaced. If the system pressure is too low, run the engine briefly to build up fuel pressure, then turn the engine off and quickly pinch shut the blue fuel return line. If the pressure holds, then the pressure regulator is faulty and should be replaced.

Fig. 7-16. Digifant II fuel rail showing location of sealing screw for service port. Service port is used for fuel pressure testing.

If the pressure still drops, check for leaks in the fuel system. If no leaks are found, the fault may be due to a leaky fuel injector, a faulty fuel injector O-ring, or a faulty fuel pump check valve.

To conclude the fuel pressure testing, disconnect the gauge from the fuel rail service port and catch any spilling fuel with a clean rag. Install the sealing screw with a new seal.

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