Fuel System Diagnosis

Fuel System Diagnosis

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.

Circuit/System Description

The engine control module (ECM) supplies a high voltage signal to the fuel pump control module when the ECM detects that the ignition is ON. The high voltage signal from the ECM to the fuel pump control module remains active for 2 s, unless the engine is in Crank or Run. While this signal is being received, the fuel pump control module 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.

The fuel tank stores the fuel supply. An electric turbine style fuel pump attaches to the fuel tank module inside the fuel tank. The fuel pump supplies fuel through the fuel feed pipe to the high pressure fuel pump. The fuel pump also supplies fuel to a venturi pump located on the bottom of the fuel tank module. The function of the venturi pump is to fill the fuel tank module reservoir. The fuel tank module contains a reverse flow check valve. The check valve maintains fuel pressure in the fuel feed pipe in order to prevent long cranking times.

Reference Information
Description and Operation

Fuel System Description (Fuel System Description)
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

Programming and Relearning (Programming and Relearning)for scan tool information
Special Tools

* CH 48027 - Digital Pressure Gage
* CH 48027-100 - Digital Pressure Gage
* 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, 580 kPa (84 psi).

1. Ignition OFF, all accessories OFF, install a CH 48027 - Gage or a CH 48027-100 - Gage.
2. Ignition ON, engine OFF, command the fuel pump ON with a scan tool. Verify the fuel pressure is between 345-414 kPa (50-60 psi) and does not decrease greater 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 gage while the fuel pump is operating. Verify the fuel pressure is between 345-414 kPa (50-60 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
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

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

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

1. Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief (With CH-48027) (Fuel Pressure Relief (With CH-48027))Fuel Pressure Relief (Without CH-48027) (Fuel Pressure Relief (Without CH-48027)).
2. Disconnect the chassis fuel feed hose from the engine compartment fuel pipe.
3. Install the J 37287 - Shut-Off Adapter between the chassis fuel hose and the engine compartment fuel pipe.
4. Open the valve on the J 37287 - Shut-Off Adapter.
5. Ignition ON, command the fuel pump ON with a scan tool and bleed the air from the CH 48027 - Gage or the CH 48027-100 - Gage.
6. Command the fuel pump ON with a scan tool.
7. Close the valve on the J 37287 - Shut-Off Adapter.
8. Monitor the fuel pressure for 1 min.

• If the fuel pressure does not decrease greater 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 greater than 14 kPa (2 psi) in 5 min.

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

4. Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH 48027 - Gage or the CH 48027-100 - Gage. 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 fuel tank module.

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

5. 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 Feed Pipe Replacement (LLT) (Fuel Feed Pipe Replacement)
* Fuel Line Replacement - Chassis (Service and Repair)
* Fuel Pump Fuel Feed Hose Replacement (Fuel Pump Fuel Feed Hose Replacement)
* Fuel Tank Module Replacement (Service and Repair)