Fuel System Diagnosis

Fuel System Diagnosis (LS9)

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

System Description

The engine control module (ECM) supplies a high voltage signal to the fuel pump control module (FPCM) when the ECM detects that the ignition is ON. The high voltage signal from the ECM to the FPCM remains active for two seconds, unless the engine is in Crank or Run. While this signal is being received, the FPCM closes the ground switch of the fuel pump and also supplies a varying voltage to the fuel tank pump module in order to maintain the desired fuel rail pressure.

The fuel system is an electronic returnless on-demand design. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal temperature of the fuel tank results in lower evaporative emissions.

Two fuel tanks store the fuel supply. An electric screw style fuel pump attaches to the fuel tank module inside the left fuel tank. The fuel pump supplies high pressure fuel through the fuel filter and the fuel feed pipe to the fuel injection system. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pump also supplies fuel to a Venturi pump located on the bottom of the left fuel tank module. The function of the Venturi pump is to fill the left fuel tank module reservoir. The primary fuel pressure regulator, a part of the left fuel tank module, maintains the correct fuel pressure to the fuel injection system. The left fuel tank module contains a reverse flow check valve. The check valve, the primary fuel pressure regulator, and the secondary fuel pressure regulator maintain fuel pressure in the fuel feed pipe and the fuel rail in order to prevent long cranking times.

The fuel pump also supplies a small amount of pressurized fuel through the auxiliary fuel feed pipe to the siphon jet pump inside the right fuel tank. The pressurized fuel creates a Venturi action inside the siphon jet pump. The Venturi action causes the fuel to be drawn out of the right fuel tank. The fuel transfers from the right fuel tank to the left fuel tank through the auxiliary fuel return pipe. The auxiliary fuel return pipe inside the left fuel tank contains an anti-siphon hole in order to prevent fuel from siphoning from the left fuel tank into the right fuel tank. Both the auxiliary fuel feed pipe and the auxiliary fuel return pipe are located inside the convoluted stainless steel crossover hose.

The right fuel tank module contains a secondary fuel pressure regulator. The secondary fuel pressure regulator has a lower set point than the primary regulator in order to allow fuel to flow to the siphon jet pump on the right fuel tank module. When the engine is shut off, the pressure in the feed pipes immediately drops to the secondary regulator set point. This prevents the siphon jet pump from operating and in turn prevents the equalization of the left and right fuel tanks. The secondary fuel pressure regulator maintains fuel pressure in the auxiliary fuel feed pipe which reduces the time to prime the siphon jet pump. The pressurization also reduces fuel vaporization and boiling in the auxiliary fuel feed pipe.

Reference Information
Description and Operation

Fuel System Description (LS9) (Description and Operation)Fuel System Description (LS3 or LS7) (Description and Operation)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information
Special Tools

* CH 48027 - Digital Pressure Gauge
* J 37287 - Fuel Line Shut-off Adapter

Circuit/System Verification

Note:
* Repair all fuel system related DTCs before performing this diagnostic.
* Inspect the fuel system for damage or external leaks before proceeding.
* Verify that adequate fuel is in the fuel tank before proceeding.
* The fuel pump may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* Before proceeding with this test review the User Manual CH 48027-5 for Safety Information and Instructions.
* DO NOT perform the Fuel System Diagnosis if the engine coolant temperature (ECT) is above 60°C (150°F). High fuel pressure readings may result due to hot soak fuel boiling. With the engine OFF, the fuel pressure may increase to the setting of the pressure relief regulator valve, 550 kPa ± 5 % (80 psi ± 5 %).

1. Ignition OFF, all accessories OFF, install a CH 48027 - Gauge. Refer to Fuel Pressure Gauge Installation and Removal (Testing and Inspection).
2. Ignition ON, engine OFF, command the fuel pump ON with a scan tool. Verify the fuel pressure is between 345-427 kPa (50-62 psi) and does not decrease more than 34 kPa (5 psi) in 1 min.

Circuit/System Testing

Note:
* The fuel pump may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* DO NOT start the engine.

1. Ignition ON, engine OFF, command the fuel pump ON with a scan tool and observe the fuel pressure gauge while the fuel pump is operating. Verify the fuel pressure is between 380-427 kPa (55-62 psi).

• If the fuel pressure is less than the specified range, test, inspect, and repair the items listed below. If all items test normal, replace the fuel tank module.

* Restricted fuel feed pipe
* Restricted or plugged strainer.
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

2. Verify that the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 min.

• If the fuel pressure decreases more than the specified value, perform the following procedure:

1. Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief (Without CH 48027) (Fuel Pressure Relief (Without CH 48027))Fuel Pressure Relief (CH 48027) (Fuel Pressure Relief (CH-48027)).
2. Install the J 37287 - Adapter between the fuel feed pipe and the fuel rail.
3. Open the valve on the J 37287 - Adapter.
4. Ignition ON, command the fuel pump ON with a scan tool and bleed the air from the CH 48027 - Gauge.
5. Command the fuel pump ON and then OFF with a scan tool.
6. Close the valve on the J 37287 - Adapter.
7. Monitor the fuel pressure for 1 min.

• If the fuel pressure decreases more than 34 kPa (5 psi) within the specified time, locate and replace the leaking fuel injector(s).
• If the fuel pressure does not decrease more than 34 kPa (5 psi) within the specified time, replace the fuel tank module.

3. Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 min.

• If the fuel pressure decreases more than the specified value, replace the fuel tank module.

4. Remove the J 37287 - Adapter and CH 48027 - Gauge. Refer to Fuel Pressure Gauge Installation and Removal (Testing and Inspection).
5. Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH 48027 - Gauge. The fuel pressure should not drop off during acceleration, cruise or hard cornering.

• If the fuel pressure drops off, test, inspect, and repair the items listed below. If all items test normal, replace the primary fuel pump module.

* Restricted fuel feed pipe
* Restricted or plugged strainer
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections

6. If the fuel system tests normal, refer to Symptoms - Engine Controls (Symptoms - Engine Controls).

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Fuel Injection Fuel Rail Assembly Replacement (LS9) (Service and Repair)Fuel Injection Fuel Rail Assembly Replacement (Without LS9) (Service and Repair)
* Fuel Injector Replacement (Removal and Replacement)
* Fuel Level Sensor Replacement - Left Side (Fuel Level Sensor Replacement - Left Side)
* Fuel Level Sensor Replacement - Right Side (Fuel Level Sensor Replacement - Right Side)