Fuel Pressure: Testing and Inspection

Diagnostic Chart:

CSFI Fuel Supply 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 Central 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 34730-1A Fuel Pressure Gage in J 34730-E.
Or
- J34730-1A Fuel Pressure Gage with J 34730-250 Fuel Pressure Adapter Kit.
- To see tools refer to Tools and Equipment. Tools and Equipment

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 415-455 kPa (60-66 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 fuel pump check valve not holding.
- A partially disconnected fuel pulse dampener (pulsator).
- The fuel pressure regulator valve leaking.
- The central SFI injector and poppet valves leaking.
5. 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 very somewhat depending on the barometric pressure; however, the pressure idling should be less indicating the pressure regulator control.
16. The fuel pressure less than 415 kPa (60 psi) falls into the following 3 areas:
- A regulated pressure but less than 415 kPa (60 psi). Amount of fuel to injector OK, but pressure is too low. System will run lean and may set a DTC. Also, hard starting cold and overall poor performance or may not start at all. 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 un-drivable. 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. Refer to Step 3.

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

22. 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 420 kPa (61 psi) as the valve in the return line is partially closed.