Component Diagnostic Charts
Chart A-7 - Fuel System Electrical Diagnosis (Part 1 of 2):
Wiring Diagram for Fuel System Pressure Test.:
CHART A-7 (Part 1 of 2) FUEL SYSTEM PRESSURE TEST
Circuit Description:
The fuel pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled. Excess fuel pressure is bypassed back to the fuel tank. The fuel pump test terminal is located in the engine compartment. When the engine is stopped, the pump can be turned "ON" by applying battery voltage to the test terminal. Improper fuel system pressure may contribute to one or all of the following symptoms:
Cranks but won't run.
Code 44 or 45.
Cuts out, may feel like ignition problem.
Hesitation, Loss of power, or poor fuel economy.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Use pressure gage J-34730-1. Wrap a shop towel around the fuel pressure tap to absorb any small amount of fuel leakage that may occur when installing the gage. With ignition "ON" pump pressure is controlled by spring pressure and throttle body vacuum within the pressure regulator assembly. Pressure should not leak down after the fuel pump is shut "OFF".
2. When the engine is idling, the throttle body vacuum is high and is applied to the fuel regulator diaphragm. This will offset the spring and result in a lower fuel pressure.
3. The application of 12-14 inches Hg of vacuum to the pressure regulator should result in a fuel pressure less than step 1.
4. Pressure that leaks down may be caused by one of the following:
In-tank fuel pump check valve not holding.
Pump coupling hose leaking.
Fuel pressure regulator valve leaking,
Injector sticking open
Chart A-7 - Fuel System Electrical Diagnosis (Part 2 of 2):
CHART A-7 (Part 2 of 2) FUEL SYSTEM PRESSURE TEST
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
5. Pressure but less than specifications falls into two areas:
Regulated pressure but less than specifications; The amount of fuel to injectors is OK but pressure is too low. The system will be lean running and may set Code 44. Also, possible hard starting cold and overall poor performance.
Restricted flow causing pressure drop; Normally, a vehicle with a fuel pressure of less than 165 kPa (24 psi) at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop as pressure begins to drop rapidly.
6. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the pump test terminal, pressure should be above 517 kPa (75 psi).
7. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.