Digifant II

Although the quantity of fuel delivered to the engine is controlled primarily by how long the injectors are open, fuel pressure also has some influence. The fuel pressure created by the fuel pump is regulated by a diaphragm-type pressure regulator mounted to the end of the fuel rail.

A vacuum hose connected to the top of the regulator helps to regulate the fuel pressure in proportion to intake manifold vacuum/pressure. When the throttle is opened quickly and vacuum drops suddenly, fuel pressure increases slightly to richen the air/fuel mixture for improved throttle response. Also when the intake manifold is under boost pressure this feature will increase fuel pressure to avoid a lean air/fuel mixture.

CAUTION: Cleanliness is essential when working with Digifant fuel system components. Before disconnecting any fuel lines, throughly clean the unions. Always use clean tools.

SYSTEM PRESSURE

System pressure is the pressure value which is maintained in the system by the fuel pressure regulator. When fuel pressure from the pump exceeds the desired system pressure, the regulator opens and routes excess fuel back to the fuel tank. System pressure is not adjustable and the pressure regulator should be replaced if it does not maintain constant system pressure, or is determined to be faulty.

RESIDUAL PRESSURE

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.

FUEL PUMP AFTER-RUN SYSTEM

The purpose of the fuel pump after-run system is to reduce the chances of the fuel vaporizing, (boiling) in the fuel rail when under hood temperatures are high.

To prevent fuel vaporizing the fuel pump is used to recirculate the fuel. The pump will be switched ON two minutes after the ignition is switched OFF and the under hood temperature is above 194°F (90°C) and the fuel pressure is above 17 psi (1.2 bar). The pumps can operate for a maximum of eight minutes.

TO TEST FUEL PRESSURE

WARNING: 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.

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

Digifant Fuel Delivery Components:

1. Wrap the fuel rail with a clean rag, then loosen and remove the hose from the fuel rail service port and connect a fuel pressure gauge.
2. START the engine and allow it to idle. Fuel system pressure should be:

36 psi (2.5 bar)

3. Disconnect the vacuum hose from the top of the fuel pressure regulator. Fuel pressure should increase to:

44 psi (3.0 bar)

4. Turn the engine OFF. After ten (10) minutes, residual pressure should be at least:

29 psi (2.0 bar)

5. If the system pressure is too high, the pressure regulator is faulty and should be replaced. If the 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.

6. 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.

7. To finish the fuel pressure test, disconnect the gauge from the fuel rail service port and catch any spilling fuel with a clean rag. Reinstall the fuel hose to the end of the fuel rail.