Chart A-4B Fuel Pressure Out of Range Diagnosis

Chart A-4B:

DFI Fuel Pump Circuit:

NOTES ON FAULT TREE:

1. The fuel pressure gauge J-25400-300 or equivalent should be installed at the fuel line service fitting. Measure the fuel pressure while cranking the engine. If the fuel pressure is between 9.0 and 12.0 PSI, then refer to the fuel system diagnosis CHART A-4A.

NOTE: The DFI system requires that fuel pressure be 9.0-12.0 PSI and steady under all driving conditions. The 325400-300 0-60 PSI pressure gauge may not show small variations in fuel pressure. For a more precise and more responsive gauge reading, use the 0-15 PSI gauge head (from the essential tool gauge number J29658/BT8205) or equivalent attached to the hose with schrader valve fitting from the essential tool gauge J34730-1.

2. If the fuel pump relay or ECM were the cause of a low fuel pressure, there would be an ECM code E20 set. This step is to check for voltage supply to the fuel tank five-way connector.

3. If the voltage signal to the fuel tank connector is OK, then an open may exist between the five-way fuel tank connector and the fuel pump. If fuel pump runs with an alternative power source connected, the fuel tank unit is OK; check throttle body fuel metering assembly for cause of low pressure.

4. Checking for fuel supply system (tank, filter, pump, sender, supply line) able to deliver at least 9.0 PSI pressure or for throttle body fuel pressure regulator fault.

5. If pressure is low with fuel return line plugged, then throttle body fuel metering assembly is not at fault. A restriction or blockage may exist in the fuel supply system. The fuel supply line should be checked visually for kinks, damage, etc.; the fuel filter element can also restrict flow. Check sender tubes for restrictions, check for rubber coupler between pump and sender leaking or restricted, check for fuel strainer in tank collapsed, mispositioned or restricted. If all of the above are OK, replace the fuel pump.

6. Fuel pressure above 12.0 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, code E45 or oxygen sensor contamination resulting in code E13 accompanied by E45 may result from the too rich fuel flow. To isolate the cause of the high fuel pressure, disconnect the 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 12.0 PSI, replace the fuel metering assembly--it is unable to control pressure properly. If the fuel pressure drops into the correct pressure range of 9.0-12.0 PSI, with the fuel return line bypassed, then the fuel return line is restricted. A restricted fuel return line can be diagnosed by visually inspecting the line for kinks, damage, etc. A kink in the Teflon fuel line (braided stainless steel clad) may not be visually obvious.

7. If the fuel pump will not run with externally applied power, the fault is an electrical open in the fuel sender unit wiring to the pump, an open at the RFI suppression connector inside the tank on the pump or a faulty fuel pump. The fuel sending unit must be removed from the vehicle to check.