Chart A-6 Fuel System Diagnosis

Chart A-6:

Chart A-6 Fuel System Diagnosis (Part 2 Of 2):

Fuel System Circuit:

Fuel System:

Circuit Description
When the ignition switch is turned "ON," the Powertrain Control Module (PCM) will turn "ON" the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the PCM is receiving reference pulses. If there are no reference pulses, the PCM will shut "OFF" the fuel pump within 2 seconds after ignition "ON" or engine stops.

An electric fuel pump, attached to the fuel sender assembly (inside the fuel tank), pumps fuel through an in-line filter to the fuel passage within the lower manifold assembly. The pump is designed to provide fuel at a pressure above the regulated pressure needed by the injectors. A pressure regulator, attached to the lower manifold assembly, keeps fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

Chart Test Description
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Install fuel pressure gage. Refer to "Fuel System Pressure Test". Ignition "ON" pump pressure should be 284-325 kPa (41-47 psi). This pressure is controlled by spring pressure within the regulator assembly.

2. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on barometric pressure, however, the pressure idling should be less, indicating pressure regulator control.

3. Pressure that continues to fall is caused by one of the following:

^ In-tank fuel pump check valve not holding.
^ Leaking fuel pump connecting hose
^ Fuel pressure regulator valve leaking.
^ Injector(s) sticking open.

4. An injector sticking open can best be determined by checking for a fouled or saturated spark plug(s).

5. Fuel pressure that drops off during acceleration, cruise or hard cornering may cause a lean condition and result in a loss of power, surging or misfire. This condition can be diagnosed using a Tech 1 scan tool. If the fuel system is very lean, the oxygen sensor will stop toggling and output voltage will drop below 500 mV. Also, injector pulse width will increase.

Important
Make sure system is not operating at "Fuel Cut-Off" which may cause false readings on the scan tool.

6. Fuel pressure below 284 kPa (41 psi) may cause a lean condition and may set a DTC 44 or a DTC 32. Driveability conditions can include hard starting cold, hesitation, poor driveability, lack of power, surging or misfire.

7. Restricting fuel flow at the fuel pressure gage causes fuel pressure to build above regulated pressure. With battery voltage applied to the pump "test" terminal, pressure should rise above 325 kPa (47 psi) as the gage outlet hose is pinched.

NOTICE
Do not allow pressure to exceed 414 kPa (60 psi) as damage to the regulator may result.

8. Fuel pressure above 325 (47 psi) may cause a rich condition and may set a DTC 45. Driveability conditions can include hard starting (followed by black smoke) and a strong sulphur smell in the exhaust.

9. This test determines if the high fuel pressure is due to a restricted fuel return line or a faulty fuel pressure regulator.