Fuel Pressure: Testing and Inspection

Fig. 015 - Chart A-7A Fuel System Diagnosis.:

Fig. 016 - Wiring Diagram for Chart A-7A Fuel System Diagnosis. VIN 8:

CHART A-7A, FUEL SYSTEM DIAGNOSIS

When the ignition switch is turned "ON", the Electronic Control Module (ECM) will turn "ON" the in-tank fuel pump for 2 seconds. It will turn it on again during cranking, and keep it 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 34 to 39 psi (235-270 kPa) with engine not running or at WOT. Excess fuel is then returned to the fuel tank.
1. Use a pressure gage. 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 presure should be 235-270 kPa (34-39 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.

The idle pressure will vary somewhat depending on barometric pressure, however, the pressure idling was less indicating pressure regulator control.

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(s) sticking open (including cold start injector).
An injector sticking open can best be determined by checking for a fouled or saturated spark plug.
If a leaking injector can not be determined by a fouled or saturated spark plug, the following procedure can be used.
a. Remove plenum, cold start valve and remove fuel rail bolts leave fuel lines connected.
b. Reconnect cold start valve.
c. Connect a hose to valve nozzle and insert into a gasoline container.
d. Lift fuel rail out just enough to leave injector nozzles in the ports.

CAUTION: Be sure injector(s) are not allowed to spray on engine and that injector retaining clips are intact. This should be carefully followed to prevent fuel spray on engine which would cause a fire hazard.

e. Pressurize the fuel system.
if. Lift each side of rail up and observe for injector(s) leaking.

Fig. 016 - Wiring Diagram for Chart A-7A Fuel System Diagnosis. VIN 8:

Fig. 017 - Chart A-7B Fuel System Diagnosis.:

CHART A-7B, FUEL SYSTEM DIAGNOSIS

1. Pressure but less than 235 kPa (34 PSI) falls into two areas:
a. Pressure but less than 235 kPa (34 PSI). Amount of the 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.
b. 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.
2. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the ALCL pump test terminal "G", pressure should be above (414 kPa 60 PSI).
3. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.