Fuel System Diagnosis

Circuit Description
The Fuel System Diagnosis determines if the fuel system is providing the correct fuel pressure.

The following components are involved:
^ The fuel tank
^ The sending unit
^ The pump
^ The lines and the hoses
^ The pressure regulator
^ The fuel rail
^ The injectors

Also checked is the electrical operation of the fuel pump relay. This chart requires the J34730-91 Fuel Pressure Gauge.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. With the ignition switch 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
4. A fuel pressure which 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. Diagnose a lean condition using a 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 the 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.
12. 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.
15. 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.
21. Check the spark plug that is associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:
1. Remove the fuel rail, but leave the fuel lines connected to the fuel rail. Refer to Fuel Injectors and Fuel Rail Replacement.

Caution: In order to reduce the risk of fire and personal injury that may result from fuel spraying on the engine, verify that the fuel rail is positioned over the fuel injector ports. Also verify that the fuel injector retaining clips are intact.

2. Lift the fuel rail just enough in order to leave the fuel injector nozzles in the fuel injector ports.
3. Pressurize the fuel system by using the scan tool fuel pump enable.
4. Visually and physically inspect the fuel injector nozzles for leaks.