Component Diagnostic Charts
Chart A-7 - Fuel System Diagnosis (Part 1 of 2):
Fuel System Diagnosis:
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 1 OF 2)
Circuit Description:
When the ignition switch is turned "ON", the Electronic 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 HEI distributor reference pulses.
If there are no reference pulses, the ECM will shut "OFF" the fuel pump within 2 seconds after key "ON" or engine stopped.
The pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled to 179 to 317 kPa (26 to 46 psi). Excess fuel is then returned to the fuel tank.
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. Ignition "ON" pump pressure should be 255-298 kPa (37-43 psi). This pressure is controlled by spring pressure within the regulator assembly.
2. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel regulator diaphragm.
This will offset the spring and result in a lower fuel pressure, 228-278 kPa (33-40 psi). This idle pressure will vary somewhat, depending on barometric pressure, however, the pressure idling was less, indicating pressure regulator control. If fuel is observed in vacuum hose to pressure regulator, the regulator is faulty and must be replaced.
3. Pressure that continues to fall is 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 Diagnosis (Part 2 of 2):
CHART A-7 - FUEL SYSTEM DIAGNOSIS (PART 2 OF 2) CONTINUED FROM CHART A-7 OR A-3
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
4. Pressure but less than 234 kPa (34 psi) falls into two areas:
^ Regulated pressure, but less than 234 kPa (34 psi). Amount of fuel to injectors OK, but pressure is too low. System will be lean running and may set Code 44. Also, 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.
5. 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).
6. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.