Fuel System Pressure Test

DFI Fuel Pump/Injector Circuit:

DFI Chart A-4, Fuel System Diagnosis:

CHART A-4, FUEL SYSTEM DIAGNOSIS

1. Fuel system diagnosis should begin by determining if the fuel pump is operating properly. The fuel pressure gage J-25400-300 should be installed at the fuel line service fitting. Measure the fuel pressure while cranking the engine. If the fuel pressure is between 9 and 12 PSI, then refer to the INJECTOR SYSTEM DIAGNOSIS.
2. If the fuel pressure is less than 9 PSI and if there are no codes set, then the fuel pump relay and the ECM are OK. Turn the ignition OFF, raise vehicle and disconnect the 6-way weatherpack connector which is mounted beneath the rear seat. Probe harness pin F with the J-29125 voltmeter as the ignition is turned on. If the voltage on pin F does not go to 12 volts for 2 seconds, then repair circuit #120 for an open.
3. If the fuel pump signal at the weather pack connector is OK, then a fuel delivery system problem exists. Disconnect the fuel return line at the throttle body and install a plug in the throttle body opening. This procedure eliminates the pressure regulator from the fuel delivery circuit. If the pump is able to produce more than 9 PSI with the pressure regulator out of the circuit, replace the fuel metering assembly because it is holding the pressure below 9 PSI.
4. If the fuel pressure remains below 9 PSI with the return line plugged, a restriction or a leak may exist in the fuel supply line. A restricted fuel supply line should be checked visually for kinks, damage, etc.; the fuel filter element can also restrict fuel flow. Check the rubber coupler in the fuel tank between the pump and fuel sender for cracks, leaks, swelling or loose fittings. If no leaks or restrictions are found, replace the fuel pump.
5. Fuel pressure above 12 PSI is caused either by a malfunction of the pressure regulator or by a restriction in the fuel return line. It should be noted that a secondary condition of spark plug fouling may result from the rich fuel flow. To isolate the cause of the high fuel pressure, disconnect the fuel return line at the throttle body and connect a suitable fitting to the throttle body which will accept a length of flexible rubber fuel hose. Insert the other end of the hose into a suitable fuel container and observe the fuel pressure as the ignition switch is turned on. If the fuel pressure remains above 12 PSI, replace the fuel metering assembly since it is unable to control the pressure properly. If the fuel pressure drops into the correct pressure range of 9-12 PSI, the fuel return line is restricted. A restricted fuel return line can be located by visually inspecting the line for kinks, damage, etc. A kink in the teflon line (flexible steel clad) may not be visually obvious.