Fuel Pressure: Testing and Inspection

Fig. 10 Checking Fuel Pressure at Fuel Rail:

WARNING: Release fuel system pressure before testing removing or disconnecting fuel system hoses or components. Service vehicles in well ventilated areas and avoid ignition sources. Never smoke while servicing the vehicle.

1. Release fuel system pressure before disconnecting a fuel hose. Perform fuel system pressure release.
2. Remove protective cover from service valve on the fuel rail.
3. Connect Fuel Pressure Gauge C-4799B to fuel rail service valve. (Fig. 10).

CAUTION: When using the ASD Fuel System Test, the Auto Shutdown (ASD) relay and fuel pump relay remain energized for 7 minutes or until the test is stopped, or until the ignition switch is turned to the Off position.

4. Place the ignition key in the ON position. Using the DRB tester, access ASD Fuel System Test. The ASD Fuel System Test will activate the fuel pump and pressurize the system.
5. If the gauge reading equals the specification listed in the Fuel System Pressure chart, further testing is not required. If pressure is not correct, record the pressure and remove gauge.
6. Ensure fuel does not leak from fuel rail service valve. Install protective cover onto fuel rail service valve.
7. If pressure is below specifications, proceed to Fuel System Pressure Below Specifications. If pressure is above specifications, proceed to Fuel System Pressure Above Specifications.

Fuel System Pressure Below Specifications
1. If the fuel pressure reading was below specifications, test the system according to the following procedure.
a. Perform Fuel Pressure Release procedure.

Fig. 12 Checking Fuel Pressure Between Pump And Filter:

b. Install Fuel Pressure Gauge C-4799 and Fuel Pressure Adapter 6631 in the fuel supply line between the fuel tank and fuel filter (Fig. 12).
c. Using the DRB, with the ignition key in the ON position, repeat the ASD Fuel System Test.

WARNING: Fuel system pressure must be released before a fuel system hose or component is disconnected.

- If pressure is at least 5 psi higher than reading recorded, replace fuel filter.
- If no change is observed, gently squeeze return hose. If pressure increases, replace pressure regulator. If the gauge reading does not change when the return hose is squeezed, the problem is either a plugged inlet strainer or defective fuel pump.

Fuel System Pressure Above Specifications
1. If the fuel pressure reading was above specifications test the system according to the following procedure.
a. Perform Fuel Pressure Release procedure.
b. Install Fuel Pressure Gauge C-4799 and Fuel Pressure Adapter 6631 in the fuel supply line (Fig. 11) between the fuel tank and fuel filter.
c. Remove the fuel return line hose from the fuel pump at fuel tank. Connect Fuel Pressure Test Adapter 6668 to the return line. Place the other end of adapter 6668 into an approved gasoline container (minimum 2 gallon size). All return fuel will flow into container.
d. Using the DRB, with the ignition key in the ON position, repeat the ASD Fuel System Test.

WARNING: Fuel system pressure must be released before a fuel system hose or component is disconnected.

Fig. 13 Fuel Return Line Adapter:

- If pressure is now correct, replace fuel pump assembly.
- If pressure is still above specifications, remove fuel return hose from chassis fuel tubes (at engine). Attach Fuel Pressure Test Connect Adapter 6668 to the fuel return tube and place other end of hose in clean container (Fig. 13). Repeat test. If pressure is now correct, check for restricted fuel return line. If no change is observed, replace fuel pressure regulator.

Without vacuum applied to the regulator, the fuel system operates at 379 kPa (55 PSI). With the engine idling and manifold vacuum applied to the regulator, the system operates at approximately 317 kPa (46 PSI). Fuel system pressure varies with different amounts of manifold vacuum applied to the regulator.