Fuel Pressure: Testing and Inspection
Fuel System With Fuel Gage Attached:
Circuit Description
When the ignition switch is turned 'ON", the Powertrain Control Module (PCM) will turn "ON" the in-tank fuel pump. The in-tank fuel pump 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 turn the in-tank fuel pump "OFF" 2 seconds after the ignition switch is turned "ON" or 2 seconds after the engine stops running.
The in-tank fuel pump is an electric pump within an integral reservoir. The in-tank fuel pump supplies fuel through an in-line fuel filter to the fuel passage within the lower manifold assembly. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the lower manifold assembly, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate fuel return pipe.
Table A-6 - Fuel System Diagnosis (1 Of 4):
Table A-6 - Fuel System Diagnosis (2 Of 4):
Table A-6 - Fuel System Diagnosis (3 Of 4):
Table A-6 - Fuel System Diagnosis (4 Of 4):
Test Description
Numbers below refer to the step numbers on the Diagnostic Table.
1. The On Board Diagnostics (OBD) System Check prompts the Technician to complete some basic checks and store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. This information on the scan tool can be referred to later.
2. To relieve the fuel pressure, refer to the Fuel Pressure Relief Procedure.
Fuel Pressure Release
With the ignition switch "ON" and the fuel pump running, the fuel pressure indicated by the fuel pressure gauge should be 284-325 kPa (41-47 psi).
3. A fuel system that can not maintain a constant fuel pressure has a leak in one or more of the following areas:
- The Fuel Pump Check Valve.
- The Fuel Pump Connecting Hose.
- The Valve or Valve Seat within the Fuel Pressure Regulator.
- The Fuel Injectors.
4. Fuel pressure that drops-off during acceleration, cruise, or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. A lean condition can be diagnosed using a scan tool. If an extremely lean condition occurs, the Oxygen Sensor(s) will stop toggling. The oxygen sensor output voltage(s) will drop below 500 mV. Also, the Fuel Injector Pulse Width will increase.
IMPORTANT: Make sure the fuel system is not operating in the "Fuel Out-Off Mode." This can cause false indications by the scan tool.
7. When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in Step 2 with the engine "OFF."
11. A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause a DTC P0132 or a DTC P0172 to set. Driveability conditions associated with rich conditions can include hard starting (followed by black smoke) and a strong sulfur smell in the exhaust.
12. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.
15. A lean condition may result from the fuel pressure being below 284 kPa (41 psi). A lean condition may cause a DTC P0131 or a DTC P0171 to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring.
16. Restricting the fuel return pipe causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the system, the fuel pressure should rise above 325 kPa (47 psi) as the gauge outlet hose is pinched.
CAUTION: Do Not allow the fuel pressure to exceed 414 kPa (60 psi). Fuel pressure in excess of 414 kPa (60 psi) may damage the fuel pressure regulator.
22. Check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking.
24. The fuel pressure regulator filter screen is designed to trap any contaminants introduced during engine assembly. If the fuel pressure regulator screen is dirty, it can be removed with a small pick and discarded without potential harm to the fuel pressure regulator.