Fuel System Diagnosis

Fuel System Diagnosis (FPCM)

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provide an overview of each diagnostic category.

Circuit/System Description

The engine control module (ECM) supplies a high voltage signal to the fuel pump flow 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.

Circuit/System Verification

Important:
* 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 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.

Ignition ON, command the fuel pump ON with a scan tool. You should hear the fuel pump turn ON and OFF.

• If the fuel pump does not operate, refer to Fuel Pump Electrical Circuit Diagnosis (with LMG, LC9, or LY6) Fuel Pump Electrical Circuit Diagnosis (FPCM). Component Tests and General Diagnostics Component Tests and General Diagnostics

1. Ignition OFF, all accessories OFF, install the CH-48027. Refer to Fuel Pressure Gage Installation and Removal. Fuel Pressure Gage Installation and Removal
2. Ignition ON, command the fuel pump relay ON with a scan tool. Verify the fuel pressure is between 345-414 kPa (50-60 psi) and remains steady for 5 minutes.

Circuit/System Testing

Important:
* The fuel pump relay may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.

* DO NOT start the engine.

Ignition ON, command the fuel pump relay 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 greater than the specified range, replace the fuel pump.
• If the fuel pressure is less than the specified range, remove the fuel tank and test, inspect, and repair the items listed below. If all items test normal, replace the fuel pump.

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

Important: The fuel pressure may vary slightly when the fuel pump stops operating. After the fuel pump stops operating, the fuel pressure should stabilize and remain constant.

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

• 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 (With CH 48027) Fuel Pressure Relief (Without CH 48027).
2. Install the J 37287 between the fuel feed pipe and the fuel rail.
3. Open the valve on the J 37287.
4. Ignition ON, command the fuel pump ON with a scan tool and bleed the air from the fuel pressure gage.
5. Close the valve on the J 37287.
6. Verify that the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 minute.

• If the fuel pressure drops, locate and replace the leaking fuel injector.

7. If the fuel system test normal, replace the fuel pump.

2. 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 minutes.

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

3. Remove the J 37287 and CH-48027.
4. Operate the vehicle within the conditions of the customers concern while monitoring the fuel trim and O2 parameters with a scan tool. The scan tool parameters should not indicate a lean condition.

• If the scan tool parameters indicate a lean condition, test for a restricted fuel feed pipe, restricted fuel filter, or poor connections at the harness connectors and ground circuits of the fuel pump. If all test normal, replace the fuel pump.

5. If the fuel system components test normal, refer to Symptoms - Computers and Control Systems. - Symptoms - Engine Controls

Repair Instructions

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

* Fuel Injector Replacement
* Fuel Tank Fuel Pump Module Replacement (15 Reg Cab w/26 gal Tank) Fuel Tank Fuel Pump Module Replacement (15 Ext/Crew Cab w/26 gal Tank) Fuel Tank Fuel Pump Module Replacement (25 Ext/Crew Cab w/26 gal Tank) Fuel Tank Fuel Pump Module Replacement (15/25 Reg/Ext/Crew Cab w/34 gal Tank) Fuel Tank Fuel Pump Module Replacement (35 Reg/Ext/Crew Cab w/34 gal Tank)