Fuel System Diagnosis (LC8FHZ)
Fuel System Diagnosis (LC8\FHZ)
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) enables the fuel pump relay when the ignition switch is turned ON. The ECM will disable the fuel pump relay within 4 s unless the ECM detects ignition reference pulses. The ECM continues to enable the fuel pump relay as long as ignition reference pulses are detected. The ECM disables the fuel pump relay within 2 s if ignition reference pulses cease to be detected and the ignition remains ON.
When the fuel pump relay is commanded ON the CNG control module supplies battery voltage and ground to the 4 compressed natural gas (CNG) high pressure lock-off (HPL) solenoid valves and the high pressure regulator (HPR) solenoid. Each HPL solenoid valve will energize opening the high pressure CNG tanks. With the tanks open, CNG will be supplied through the high pressure fuel lines to the high pressure regulator at pressures up to 24821 kPa (3600 psig). The CNG control module also supplies a ground and battery voltage to the high pressure regulator. When energized the high pressure regulator maintains the outlet low pressure between 620-758 kPa (90-110 psig) that is supplied to the fuel injectors through the low pressure fuel lines and fuel rails.
Reference Information
Description and Operation
Fuel System Description (LC8\K07\UFP) Fuel System Description (LC8K07UFP)Fuel System Description (LC8\FHZ) Fuel System Description (LC8FHZ)Fuel System Description (Without LC8) Fuel System Description (Without LC8)
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 Control Module References for scan tool information
Special Tools
CH-48027 Digital Pressure Gauge
Circuit/System Verification
Note:
* Repair all fuel system related DTCs before performing this diagnostic, unless instructed here from a DTC diagnostic.
* Inspect the fuel system for damage or external leaks before proceeding.
* Verify that adequate fuel is in the fuel tanks before proceeding.
* 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 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 above 930 kPa(135 psi) the pressure relief valve set point is 1379 kPa(200 psi).
1. Verify all four CNG fuel tank manual shut-off valves are OPEN.
2. Verify the 1/4 turn manual shut off valve is OPEN.
3. Ignition OFF, all accessories OFF, install a CH-48027. Refer to Fuel Pressure Gauge Installation and Removal Fuel Pressure Gauge Installation and Removal.
4. Ignition ON, command the fuel pump relay ON with a scan tool. Verify the fuel pressure is between 620-758 kPa (90- 110 psi)
• If greater than 896 kPa (130 psi)
Replace the fuel pressure regulator. Refer to Compressed Natural Gas High Pressure Regulator Replacement Compressed Natural Gas High Pressure Regulator Replacement.
• If less than the specified value
1. Ignition ON, command the fuel pump relay ON several times.
2. Verify the fuel pressure is less than 620 kPa (90 psi).
• If greater than the specified value, test or replace the CNG fuel filter. Refer to Compressed Natural Gas High Pressure Regulator Filter Replacement Compressed Natural Gas High Pressure Regulator Filter Replacement.
• If less than the specified value, refer to Circuit/System Testing.
• If within the specified value
5. Ignition ON, command the fuel pump relay ON with a scan tool. Ignition OFF, verify the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 min.
• If the fuel pressure decreases greater than the specified value
Start the vehicle and inspect for CNG fuel leaks with the J-45878 - Combustible Gas Detectoror an equivalent leak detector.
• If the fuel pressure decreases less than the specified value
6. Engine idling, verify the fuel pressure is between 620-758 kPa (90- 110 psi)
• If less than the specified value
Refer to Circuit/System Testing
• If fuel pressure is within the specified value
7. Operate the vehicle within the conditions of the customers concern. Verify the fuel pressure is between 620-758 kPa (90- 110 psi).
• If less than the specified value
Refer to Circuit/System Testing
• If within the specified value
8. All OK
Circuit/System Testing
1. Ignition OFF, disconnect the X2 harness connector at the CNG control module. Connect a test lamp between the fuel pump relay component control circuit terminal D and ground.
2. Ignition ON, command the fuel pump relay ON with a scan. Verify the test lamp illuminates.
• If the test lamp does not illuminate
Refer to Fuel Pump Electrical Circuit Diagnosis (FPCM) Fuel Pump Electrical Circuit Diagnosis (FPCM)Fuel Pump Electrical Circuit Diagnosis (LC8\K07\UFP) Fuel Pump Electrical Circuit Diagnosis (LC8K07UFP)Fuel Pump Electrical Circuit Diagnosis (LC8\FHZ) Fuel Pump Electrical Circuit Diagnosis (LC8FHZ)Fuel Pump Electrical Circuit Diagnosis (Without FPCM) Fuel Pump Electrical Circuit Diagnosis (Without FPCM).
• If the test lamp illuminates
3. Test for 10 ohms or less between the CNG control module ground circuit terminal A and ground.
• If 10 ohms or greater
1. Ignition OFF.
2. Test for less than 2 ohms in the ground circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms , repair the open/high resistance in the ground connection.
• If less than 10 ohms
4. Ignition OFF, connect the CNG control module X2 harness connector.
5. Disconnect all 4 of the HPL solenoid valve harness connectors.
6. Disconnect the fuel pressure regulator harness connector.
Note:
Perform steps 7-11 on each of the HPL Solenoid valves.
7. Test for 10 ohms or less between the HPL solenoid ground circuit terminal B and ground.
• If 10 ohms or greater
1. Ignition OFF, disconnect the CNG control module X1 harness connector.
2. Test for 2 ohms or less between the HPL solenoid valve ground circuit terminal B and the appropriate CNG control module X1 connector terminal. Refer to Engine Controls Schematics [1][2]Electrical Diagrams
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the CNG control module.
• If less than 10 ohms
8. Connect a test lamp between the HPL solenoid valve supply circuit terminal A and ground circuit terminal B.
9. Ignition ON, command the fuel pump relay ON, Verify the test lamp illuminates.
• If the test lamp does not illuminate
1. Ignition OFF, disconnect the CNG control module X1 harness connector.
2. Test for 2 ohms or less between the HPL solenoid valve supply circuit terminal A and the appropriate CNG control module X1 connector terminal. Refer to Engine Controls Schematics [1][2]Electrical Diagrams
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the CNG control module.
• If the Test lamp illuminates
10. Test for 13-18 ohms between terminals A and terminal B of the HPL solenoid valve.
• If not within the specified range
Replace the appropriate HPL solenoid valve.
• If within the specified range
11. Test for infinite resistance between the HPL solenoid valve terminal A and ground.
• If less than infinite resistance
Replace the appropriate HPL solenoid valve.
• If infinite resistance
12. Test for less than 10 ohms between the fuel pressure regulator ground circuit terminal A and ground.
• If 10 ohms or greater
1. Ignition OFF, disconnect the CNG control module X1 harness connector.
2. Test for 2 ohms or less between the fuel pressure regulator ground circuit terminal B and the CNG control module X1 connector terminal K.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the CNG control module.
• If less than 10 ohms
13. Connect a test lamp between the fuel pressure regulator supply circuit terminal A and ground circuit terminal B.
14. Ignition ON, command the fuel pump relay ON, Verify the test lamp illuminates.
• If the test lamp does not illuminate
1. Ignition OFF, disconnect the CNG control module X1 harness connector.
2. Test for 2 ohms or less between the fuel pressure regulator supply circuit terminal A and the CNG control module X1 connector terminal C.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the CNG control module.
• If the Test lamp illuminates
15. Test for 13-18 ohms between terminals A and terminal B of the fuel pressure regulator.
• If not within the specified range
Replace the fuel pressure regulator.
• If within the specified range
16. Test for infinite resistance between the fuel pressure regulator terminal A and ground.
• If less than infinite resistance
Replace the fuel pressure regulator.
• If infinite resistance
17. Refer to High Pressure Lock-out Solenoid Valve Test
High Pressure Lock-out Solenoid Valve Test
Note:
Repeat the steps 6-12 for each or the four CNG tanks
1. Ignition OFF, all accessories OFF, install a CH-48027. Refer to Fuel Pressure Gauge Installation and Removal Fuel Pressure Gauge Installation and Removal.
2. Close the 1/4 Turn Shut Off Valve.
3. Start and run the engine until is stalls, Vent the remaining fuel pressure.
4. Verify that the fuel pressure is less than 34 kPa (5 psi).
• If 34 kPa (5 psi) or greater
Replace the 1/4 Turn Shut Off Valve.
• If less than 34 kPa (5 psi)
5. Open the 1/4 Turn Shut Off Valve.
6. Close all of the manual shut off valves on the CNG fuel tanks.
7. Start and run the engine until is stalls, Vent the remaining fuel pressure.
8. Verify that the fuel pressure is less than 34 kPa (5 psi)
• If 34 kPa (5 psi) or greater
1. Vent the fuel
2. Determine which manual valve is leaking
• If less than 34 kPa (5 psi)
9. Open the manual shut off valve on one tank
10. Verify that the fuel pressure is less than 34 kPa (5 psi)
• If 34 kPa (5 psi) or greater
Replace the appropriate high pressure lock-out solenoid valve assembly.
• If less than 34 kPa (5 psi)
11. Command the fuel pump relay on with a scan tool
12. Verify the fuel pressure is between 620-758 kPa (90- 110 psi)
• If not between 620-758 kPa (90- 110 psi)
Replace the appropriate high pressure lock-out solenoid valve assembly.
• If between 620-758 kPa (90- 110 psi)
13. All OK, Repeat steps 6-12 for the remaining CNG fuel tanks.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Compressed Natural Gas Control Module Replacement Service and Repair
* Compressed Natural Gas High Pressure Regulator Filter Replacement Compressed Natural Gas High Pressure Regulator Filter Replacement
* Compressed Natural Gas High Pressure Regulator Replacement Compressed Natural Gas High Pressure Regulator Replacement
* CNG High Pressure Shutoff Valve Replacement Service and Repair
* CNG Tank High Pressure Shutoff Valve Solenoid Replacement Service and Repair