Fuel Pressure: Testing and Inspection
CIRCUIT DESCRIPTIONWhen the ignition is turned ON, the Electronic Control Module (ECM) compares the engine coolant temperature to an internal clock to determine if a fuel pump prime is required. When the ECM calculates that a fuel prime is necessary, the fuel pump relay will be turned ON for 2 seconds. The pump remains ON as long as the engine is cranking or running and the ECM is receiving reference pulses. If there are no reference pulses, the ECM shuts the fuel pump OFF within 2 seconds after the ignition is switched OFF. If the ECM calculations indicate that the fuel system still has adequate residual pressure, a prime will not occur.
An electric fuel pump supplies the fuel through an in-line filter to the fuel rail and injectors. The pump is attached to the fuel sender assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe. The fuel system does not adjust fuel pressure to compensate for manifold vacuum. The port on the fuel pressure regulator is an atmospheric vent only. This port is connected to the bottom of the throttle body and vented in front of the throttle plates.
TEST DESCRIPTION
Steps 1-8:
Steps 9-16:
Steps 17-23:
The numbers below refer to the step numbers on the diagnostic table.
2. With the ignition ON and the fuel pump running, the fuel pressure that is indicated by the fuel pressure gauge should be 275-313 kPa (40-46 psi). This pressure is controlled by spring pressure inside of the fuel pressure regulator.
3. A fuel system that cannot maintain a constant fuel pressure has a leak in one or more of the following areas:
^ The fuel pump check valve
^ The valve or valve seat within the fuel pressure regulator
^ The fuel injectors
^ The fuel feed line
^ The fuel feed line in the fuel tank
4. Fuel pressure which drops off during acceleration, cruise, or hard cornering may set a DTC P1460 if the fuel level is low it may also cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. Diagnose a lean condition using a scan tool.
10. A rich condition may result from the fuel pressure being above 313 kPa (46 psi). A rich condition may cause DTC P0172 or DTC P0175 to set. Driveability conditions associated with rich conditions can include hard starting, followed by black smoke, and a strong sulfur smell in the exhaust.
A lean condition may result from the fuel pressure being below 275 kPa (40 psi). A lean condition may cause DTC P0171 or DTC P0174 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.
14. Restricting the fuel return line with the J 37287 fuel line shut-off adaptor causes the fuel pressure to rise above the regulated fuel pressure. With the fuel pump enabled using a scan tool, the fuel pressure should rise above 313 kPa (46 psi) as the valve on the fuel line shut-off adaptor connected to the fuel return line becomes partially closed.
18. If the fuel pressure regulator is leaking fuel internally there will be signs of fuel at this connection. The hose that is connected to the fuel pressure regulator does NOT have vacuum applied to it. This is an ATMOSPHERIC VENT ONLY. It is connected to the bottom of the throttle body and is vented in front of the throttle plates.