Diagnostic Procedures

Chart A-7: 1 Of 2:

Chart A-7: 2 Of 2:

Fuel System Fitted With Test Equipment:

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

An electric fuel pump, attached to the fuel sender assembly (inside the fuel tank), supplies fuel through an in-line filter to the Throttle Body Injection (TBI) unit. The pump is designed to provide sufficient fuel flow for all engine demands. A pressure regulator, attached to the fuel meter cover, 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
Numbers below refer to circled numbers on the diagnostic chart.
(Part 1 of 2)
1. Connect fuel pressure gage as shown in accompanying image. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the gage. The fuel pump "test" connector is located on the passenger side of the engine compartment. Fuel pressure should be noted with the fuel pump running. When fuel pump stops running, fuel pressure will drop immediately through a constant bleed passage machined in the fuel meter cover.

2. This step checks the operation of the fuel pump relay circuit. Observing the fuel pressure gage when ignition is cycled "ON," pressure should build when fuel pump is running and drop when pump stops.

3. This check will verify the operation of the fuel pump check valve. A leaking check valve will cause fuel in the pressure (feed) line to drain back to the tank and result in long crank times. The amount of pressure is not important as long as some pressure is maintained.

(Part 2 of 2)
4. 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 300 mV. Also, injector pulse width will increase.

5. Fuel pressure below 62 kPa (9 psi) may cause a lean condition and may set a Diagnostic Trouble Code (DTC) 44. Driveability conditions can include hard starting cold, hesitation, poor driveability, lack of power, surging or misfire.

6. Restricting fuel flow in the fuel return line as directed causes fuel pressure to build above regulated pressure. With battery voltage applied to the pump "test" connector, pressure should rise above 90 kPa (13 psi) as the shut-off valve is partially closed.

7. Fuel pressure above 90 kPa (13 psi) may cause a rich condition any may set a DTC 45. Driveability conditions can include hard starting (following by black smoke) and a strong sulphur smell in the exhaust.

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