Fuel Pressure: Testing and Inspection

FUEL SYSTEM DIAGNOSIS

Schematic:

Diagnostic Chart (Part 1 Of 6):

Diagnostic Chart (Part 2 Of 6):

Diagnostic Chart (Part 3 Of 6):

Diagnostic Chart (Part 4 Of 6):

Diagnostic Chart (Part 5 Of 6):

Diagnostic Chart (Part 6 Of 6):

CIRCUIT DESCRIPTION
When the ignition is turned ON, the Vehicle Control Module (VCM) energizes the fuel pump relay which powers the in-tank fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.

An electric fuel pump pumps the fuel through an in-line filter to the SFI unit. The pump is attached to the fuel sender assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.

2. Wrap a shop towel around the fuel pressure connection in order to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition. The fuel pressure should be 379-427 kPa (55-62 psi). This pressure is controlled by a spring pressure within the regulator assembly.

4. Fuel pressure that continues to fall is caused by one of the following conditions:
^ The in-tank modular fuel sender is not holding pressure
^ The fuel pressure regulator valve is leaking
^ An SFI injector is leaking
^ The fuel line is leaking

15. If the fuel pressure is less than 344 kPa (50 psi) at idle, it falls into the following 3 areas:
^ The pressure is regulated but less than 344 kPa (50 psi). The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run. Symptom Related Diagnostic Procedures
^ A restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
^ A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

21. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the fuel pump commanded ON, the fuel pressure should rise above 427 kPa (62 psi), as the valve in the return line is partially closed.

26. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure, however the pressure at idle should be less indicating the pressure regulator control.

29. If the fuel injectors are not grounded or leaking and the fuel pressure is not holding, the fuel pressure regulator should be inspected for a slow diaphragm leak.