Fuel Delivery and Air Induction: Testing and Inspection

Fuel System Diagnosis

Fuel System:

Circuit Description

When you turn ON the ignition, the powertrain control module (PCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the PCM is receiving reference pulses. If there are no reference pulses, the PCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The PCM will also turn ON the fuel pump for 2 seconds when the ignition is turned OFF.

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 to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injector at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

Diagnostic Aids

Tools Required
* J 34730-E Fuel Pressure Gauge
* J 34730-1A Fuel Pressure Gauge
* J 34730-250 Fuel Pressure Gauge and Adapter
* J 42965 Fuel Line Shutoff Valve (GMT600)
* J 42873-2 5/16 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 379-427 kPa (55-62 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 conditions:

* 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 344 kPa (50 psi), 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 Engine Cranks but Does 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.

21.
Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.

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 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.