Fuel Pressure: Testing and Inspection

Diagnostic Chart:

Fuel System:

CIRCUIT DESCRIPTION

When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. 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 you turn the ON the ignition or if the engine stops.

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

DIAGNOSTIC AIDS

Tools Required

- J 34730-E Fuel Pressure Gauge.

- J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.

- J 42873-1 3/8 Fuel Feed Pipe Shutoff Valve and the J 42873-2 5/16 Fuel Return Pipe Shutoff Valve.

TEST DESCRIPTION

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

2. Wrap a shop towel around the fuel pressure connection 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 385-430 kPa (56-62 psi). This pressure is controlled by a spring pressure within the regulator assembly.

4. The fuel pressure that continues to fall is caused by one of the following items:

- The in-tank modular fuel sender is not holding pressure.

- The fuel pressure regulator valve is leaking.

- An SFI injector is leaking.

- Fuel line is leaking.

15. If the fuel pressure is less than 385 kPa (56 psi), it falls into the following 3 areas:

- The pressure is regulated but less than 385 kPa (56 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. Engine Cranks, But Will Not Run

- 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.

NOTICE: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel pressure regulator.

21. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 430 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.