Chart A4B - Fuel Pressure Out Of Range
This procedure tests for fuel supply system problems that can cause incorrect fuel pressure or incorrect fuel pump operation.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Fuel system diagnosis should begin by determining if fuel pump is operating properly. Fuel Pressure Gauge (J-25400-300) should be installed at fuel line service fitting. Measure fuel pressure while cranking engine. If fuel pressure is 9-12 psi, go to CHART A4A - FUEL SYSTEM DIAGNOSIS. If necessary, Fuel Pressure Gauge (J-29658/BT8205) may be attached to service line fitting with Schrader valve fitting from Gauge Set (J-34730-1). This set up will give technician a more precise and more responsive reading.
- If fuel pump relay or ECM were cause of low fuel pressure, there would be an ECM Code EO20 set. This step is to check for voltage supply to fuel tank 5-way connector.
- If voltage signal to fuel tank connector is okay, an open may exist between 5-way fuel tank connector and fuel pump. If fuel pump runs with an alternate power source connected, the fuel tank unit is okay. Check throttle body fuel metering assembly for cause of low fuel pressure.
- Check for fuel supply system (tank, filter, pump, sender, lines) ability to deliver at least 9 psi pressure or for throttle body fuel pressure regulator fault.
- If fuel pressure is low with fuel return line plugged, throttle body fuel metering assembly is not at fault. A restriction or blockage may exist in fuel supply system. The fuel supply line should be checked visually for kinks and damage. The fuel filter element can also restrict flow. Check for proper fuel line routing, sender tubes for restrictions, check rubber between fuel pump and sending unit for leaks or restrictions, or collapsed fuel strainer in tank. If all of the above are okay, replace fuel pump.
- Fuel pressure greater than 12 psi is caused either by a malfunction of pressure regulator or by a restriction in fuel return line. It should be noted that a secondary condition of spark plug fouling, Code EO45 or oxygen sensor contamination resulting in Code EO13 accompanied by Code EO45, may result from too rich fuel flow. To isolate cause of high fuel pressure, disconnect return line at throttle body and connect a fitting to throttle body which will accept a length of flexible rubber fuel hose. Insert other end of hose into a container and observe fuel pressure as ignition is tuned on. If fuel pressure remains greater than 12 psi, replace fuel metering assembly. If fuel pressure drops into 9-12 psi range, with fuel return line by-passed, the fuel return line is restricted. A restricted fuel return line can be diagnosed by visually inspecting line for kinks or damage. A kink in the Teflon fuel line (braided stainless steel clad) may not be visually obvious.
- If fuel pump will not run with externally applied power, fault is an open circuit in fuel sending unit wiring to pump, or an open circuit at RFI suppression connector inside tank, or faulty fuel pump. The fuel sending unit must be removed from vehicle to check.