Fuel System Diagnosis RPO L35

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 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 Central 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 injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

Tools Required

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

•

A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is undrivable. 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.

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

Testing the fuel injector circuit is in Chart A-3 and additional diagnosis in Chart A-4 in Section 3.

A fuel injector or poppet nozzle which does not open, may cause a no-start condition. An injector and poppet nozzle which is stuck open, could cause loss of pressure after sitting, so long crank times would be noticed. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned OFF. These components are diagnosed in Charts A-7 and A-8.

Testing the pressure regulator circuit starts in Chart A-3 in Section 3.

If the pressure regulator in the Central MFI unit supplies pressure which is too low, poor performance could result. If the pressure is too high, excess emissions and unpleasant exhaust odor may result.

The diagnosis of the tuning valve circuit can be found in Section 3.

The tuning valve (rotary solenoid device is electrically controlled by the control module based on RPM and TP signal inputs, for accurate switching capability at optimum switching points to enable either a "split" or "single" plenum conditions.

During the low and high end of the RPM range, the control module de-energizes the valve and enables a "split" plenum condition that provides for peak torque along with increased peak horsepower.

During the mid RPM range, the control module energizes the valve, thus enabling a "single" tuning plenum condition that provides for mid RPM range peak torque.

The diagnosis of idle air control can be found in this section.

If the IAC valve is disconnected or connected with the engine running, the idle RPM may be wrong. In this case, the IAC valve may be reset by disconnecting negative battery cable for 10 seconds, then reconnect the battery cable and ignition"ON," engine "OFF" for 5 seconds, then key "OFF" for 10 seconds.

The IAC valve affects only the idle characteristics of the engine. if it is open fully, too much air will be allowed to the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle my be rough, and will not respond to engine load changes.

The minimum idle speed is set at the factory with a stop screw. The stop screw should not be adjusted.

Vacuum leaks will cause the IAC valve pintle to be "stepped" closer to the seat or to be closed against its seat in an attempt to maintain controlled idle speed.

Refer to Chart A-3 in Section 3.

An inoperative fuel pump would cause a not start condition.

A fuel pump which does not provide enough pressure can result in poor performance. Refer to Fuel System Pressure Test procedure elsewhere in this section.

Refer to Chart A-5 in Section 3, for fuel pump relay diagnosis.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn "ON" the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi).

A fuel system pressure test is part of several of the diagnostic charts and symptom checks. To perform this test, follow this procedure:

1. Turn the engine "OFF" and relieve the fuel pressure.
2. Connect the fuel pressure gage to fuel pressure connection tap. Wrap a shop towel around fitting while connecting gage to avoid spillage.
3. Connect the negative battery terminal.
4. Start the engine and observe the fuel pressure reading. It should be 400-440 kPa (58-64 psi). If not, refer to Chart A-6 in Section 3.
5. Turn the engine "OFF" and relieve the fuel pressure.
6. Remove the fuel pressure gage.
7. Reinstall the fuel pressure connection cap.
8. Reconnect the negative battery terminal.
9. Start the engine and check for fuel leaks.

Important: All central MFI component parts, with the exception of those noted below, should be cleaned in a cold immersion cleaner such as GM X-55 or equivalent.

1. All metal parts thoroughly and blow drive with shop air. Be sure that all fuel and air passages are free of dirt or burrs.
2. Mating casting surfaces for damage that could affect gasket sealing.

Service repair kits are supplied with a small vial of thread-locking compound with directions for use. if material is not available, use Loctite 262 or equivalent.