Symptoms - Engine Controls
| Important Preliminary Checks | Action |
|---|
| Before performing Symptom diagnosis | Before performing Symptom based diagnosis you must perform the Diagnostic System Check - Alternative Fuels (Engine Controls) . The System Check will determine the following: The FICM, PCM and the MIL are operating correctly. There are no DTCs stored. Scan tool data is within normal operating range. Several of the Symptom diagnostic tables require that a Visual and Physical Check be performed. Always perform a Visual and Physical Check before using a Symptom diagnostic table. |
| Visual and Physical Checks | Inspect the FICM and PCM ground connections for being clean, tight and in the correct location. Inspect the PCM for the correct CNG calibration. Program the PCM if necessary. Inspect the FICM for the proper part number. Bi-fuel (KL6) and dedicated (KL8) CNG equipped vehicles utilize unique FICM's that are not interchangeable. Inspect the vacuum hoses for leaks, restrictions and proper connection as shown on the vehicle emission control information (VECI) label. Inspect for any type of fuel leak or fuel line restriction. Inspect for air leaks at all areas of the intake manifold sealing surfaces. Inspect the ignition wires for the following conditions: Cracking Hardness Proper routing Carbon tracking Inspect vehicle wiring for the following: Proper connection Pinches Cuts |
| Symptoms | Verify the customer concern. Locate the Symptom table that indicates the customers concern. Test or Inspect for the items indicated for that symptom. The following Symptom tables are located in this section: Intermittent Conditions Hard Start Surges/Chuggles Lack of Power, Sluggishness, or Sponginess Hesitation, Sag, Stumble Cuts Out, Misses Poor Fuel Economy Poor Fuel Fill Quality Rough, Unstable, or Incorrect Idle and Stalling Dieseling, Run-On Backfire |
Important Preliminary Checks
Intermittent Conditions
| Checks | Action |
|---|
| DEFINITION: The problem may or may not turn ON the MIL or store a DTC. If a DTC was stored and the DTC diagnostic table indicates the fault is not present an Intermittent Condition is indicated. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Electrical Connections or Wiring | Electrical connections and wiring cause most intermittent conditions. Determine which circuit is suspected of having an intermittent condition. This may be indicated by DTCs with a fault not present. Inspect the suspect circuit for the following conditions: Connectors poorly mated. Terminals not fully seated in the connector - backed out. Terminals not properly formed or damaged - usually due to improper diagnostic probing procedures. Poor terminal retention. Testing for terminal retention requires the use of the J-38125 Terminal Repair Kit. Poor terminal to wire connection. Inspecting for poor terminal to wire connection requires removing the wire/terminal from the connector body. |
| Road Test | Drive the vehicle while monitoring the suspect circuit with the scan tool or the J 39200 DMM connected to the circuit. An abnormal voltage or scan tool reading indicates the suspect circuit and a possible circuit condition. |
| Scan Tool | The scan tool features that can be used to locate an intermittent conditions source include the following: Scan tool snapshot feature The scan tool snapshot feature can be triggered to capture and store engine parameters when a malfunction occurs. This stored information can be reviewed and compared to the Engine Scan Tool Data List or data values taken from a similarly equipped known good vehicle. Freeze Frame/Failure Records data feature Freeze Frame/Failure Records are stored when certain DTCs set. They typically include information to aid in reproducing the driving conditions that were present when a DTC is stored. In addition, engine parameters are also stored. This stored information can be reviewed and compared to the Engine Scan Tool Data List or data values taken from a similarly equipped known good vehicle. Freeze Frame/Failure Records data will be erased when DTC information is cleared either by scan tool clearing or by disconnection of the control modules power supply. |
| Intermittent Malfunction Indicator Lamp (MIL) | The following conditions may cause intermittent MIL operation with no DTCs stored: A defective relay, control module driven solenoid, or a switch that causes electrical system interference. Usually the symptom will occur when the faulty component is operating. The improper installation of add-on electrical devices. These can include the following: Lighting systems 2-way radios Electrical motors Ignition control (IC) wires routed near the generator or secondary ignition system wires and components. The ignition system secondary voltage shorted to ground. Poor control or ignition module grounds. The MIL control circuit intermittently shorted to ground. |
| Loss of DTC Memory | In order to test for proper DTC Memory function perform the following: Observe the Conditions For Running The DTC for the TP Sensor Circuit Low Voltage DTC. Disconnect the TP sensor connector. Operate the vehicle within the Conditions For Running The DTC. The MIL should illuminate upon completion of all the Conditions For Running The DTC. Two key cycles or drive trips may be required. A TP Sensor Circuit Low Voltage DTC should be stored in the control module and remain in memory when the ignition is turned OFF. Failure to store a DTC or for the DTC to remain in memory may indicate a faulty control module. |
| Additional Checks | Test for open diodes - A/C clutch, etc. - that may cause electrical interference. |
| Vehicle Data Recorder | The J 42598 Vehicle Data Recorder is connected to the data link connector (DLC) and sent with the customer. The captures data for later retrieval and analysis by the technician. Refer to the vehicle data recorder user instructions for more information. |
Hard Start
| Checks | Action |
|---|
| DEFINITION: The engine cranks OK, but does not start for a long time. The engine does eventually run, or may start but immediately dies. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . Make sure the vehicle operator is using the correct starting procedure. |
| Fuel System Checks | Test for proper fuel pump relay operation. Test for proper fuel system pressures. Refer to Fuel System Diagnosis . Test for an accurate fuel indication from the fuel tank pressure (FTP) sensor. |
| Sensor Checks | Test the ECT sensor for proper operation by performing the following: Allow the engine temperature to reach ambient air temperature by leaving the engine OFF for several hours - overnight for most accurate results. Turn the ignition ON while leaving the engine OFF. Using a scan tool compare the ECT and IAT sensors displayed temperature. The sensors should be within 3° C (5° F) of each other. Replace the ECT sensor or repair the ECT sensor circuit if the ECT is not near the expected value. Test the TP sensor for proper operation. With the throttle blade fully at rest the monitored TP sensor voltage on a scan tool should be LESS than 0.85 volts. If the TP sensor is not near the expected voltage check for the following conditions: A faulty TP sensor A faulty TP sensor circuit A sticking throttle shaft or linkage/cable assembly Test for proper operation of the crankshaft position (CKP) sensor. Test for proper operation of the MAP sensor - if equipped. |
Surges/Chuggles
| Checks | Action |
|---|
| DEFINITION: The engine has a power variation under a steady throttle or cruise. The vehicle feels as if speeds up and slows down with no movement of the accelerator pedal. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . Ensure the vehicle operator understands the transmissions torque converter clutch (TCC) operation, if equipped. Ensure the vehicle operator understands the A/C compressor clutch operation, if equipped. |
| Sensor Checks | Test for proper operation of the vehicle speed sensor (VSS). Test for proper operation and performance of the oxygen sensor. |
| Fuel System Checks | Inspect for a lean or rich engine condition. Using a scan tool observe the oxygen sensor output while operating the vehicle under the symptom condition. Test for proper fuel system pressures. Refer to Fuel System Diagnosis . Inspect for proper operation of the cng fuel injectors. Refer to: Fuel Injector Coil Test Fuel Injector Balance Test with Special Tool Fuel Injector Balance Test with Tech 2 Ensure that the manual shut-off valve is fully open. Inspect for restricted in-line filter. Refer to Fuel Filter Replacement (Passenger/Cargo) or Fuel Filter Replacement (Cutaway) . |
Lack of Power, Sluggishness, or Sponginess
| Checks | Action |
|---|
| DEFINITION: The engine delivers less than expected power. There is little or no increase in speed when partially applying the accelerator pedal. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . IMPORTANT: Do not compare the power output of the vehicle while operating on CNG to a vehicle operating on gasoline. Both fuels have different drive feel characteristics. Verify the customers concern by comparing the power output of a similarly equipped known good vehicle to the suspect vehicle. Inspect the air filter for proper type and for dirt or restriction. Test the transmission for proper shift pattern and downshift operation. Inspect the PCM for the correct CNG calibration. Program the PCM if necessary. |
| Fuel System Checks | Test for proper fuel system pressures. Refer to Fuel System Diagnosis . Inspect the in-line filter for restriction. Refer to Fuel Filter Replacement (Passenger/Cargo) or Fuel Filter Replacement (Cutaway) . |
| Ignition System Checks | IMPORTANT: A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. Test for proper secondary ignition system voltage output using only the J 26792 Spark Tester. IMPORTANT: More timing advance is required to operate the vehicle on CNG than on gasoline. Test for proper operation of the Knock Sensor system. |
| Exhaust System Checks | Inspect the exhaust system for a possible restriction. Restrictions can include the following: A failed catalytic converter Damaged or collapsed pipes An internally failed muffler - may show signs of heat distress |
| Engine Mechanical Checks | Test for proper engine operation by checking for the following: Correct engine compression Correct engine valve timing A worn or incorrect camshaft |
| Additional Checks | Test for proper A/C compressor clutch operation. An A/C clutch that is engaged all the time may cause this symptom. Test for proper transmission torque converter clutch (TCC) operation, if equipped. Test for proper generator operation. Inspect for proper operation of the accelerator pedal. A stretched accelerator cable may cause this symptom. |
| IMPORTANT |
|---|
| Do not compare the power output of the vehicle while operating on CNG to a vehicle operating on gasoline. Both fuels have different drive feel characteristics. |
| IMPORTANT |
|---|
| A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. |
| IMPORTANT |
|---|
| More timing advance is required to operate the vehicle on CNG than on gasoline. |
Lack of Power, Sluggishness, or Sponginess
Hesitation, Sag, Stumble
| Checks | Action |
|---|
| DEFINITION: The vehicle has a momentary lack of response when depressing the accelerator pedal. The condition can occur at any vehicle speed. The condition is usually most severe when attempting to make the vehicle move, as from a stop sign. An extremely severe condition may cause the engine to stall. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Fuel System Checks | Test for proper fuel system pressures. Refer to Fuel Pressure Relief Procedure . Inspect for low pressure during moderate or full throttle acceleration. If the fuel pressure drops below specification, there is possibly a low or high pressure regulator fault or a fuel system restriction. Test for proper EVAP purge system operation. |
| Sensor Checks | Test for proper TP sensor operation. A TP sensor that does not respond quickly and accurately to throttle plate movement may cause this symptom. Test for proper MAP sensor operation. A MAP sensor that does not respond quickly and accurately to manifold pressure changes may cause this symptom. |
| Ignition System Checks | IMPORTANT: A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. Test for proper secondary ignition system voltage output using only the J 26792 Spark Tester. Inspect for fouled spark plugs. If fouled spark plugs are indicated the reason for the fouling must be found and corrected. Test for faulty spark plug wires. Test for an open ignition system ground - if equipped. |
| Additional Checks | Review service bulletins for AF ECM or PCM programming updates. Inspect for manifold or air induction system leaks. Test for proper generator operation. |
| IMPORTANT |
|---|
| A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. |
Cuts Out, Misses
| Checks | Action |
|---|
| DEFINITION: A surging or jerking that follows engine speed, usually more pronounced as the engine load increases. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Fuel System Checks | Test for proper fuel system pressures. Refer to Fuel System Diagnosis . Inspect for proper operation of the cng fuel injectors. Refer to: Fuel Injector Coil Test Fuel Injector Balance Test with Special Tool Fuel Injector Balance Test with Tech 2 Test the high and low pressure lock-off valves/circuits for proper operation. |
| Ignition System Checks | IMPORTANT: A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. Check for proper secondary ignition system voltage output using ONLY the J 26792 Spark Tester. Inspect for fouled spark plugs. If fouled spark plugs are indicated the reason for the fouling must be found and corrected. Test for faulty spark plug wires. Test for an open ignition system ground, if equipped. |
| Engine Mechanical Checks | Test for proper engine operation by checking for the following: Correct engine compression Correct engine valve timing A worn or incorrect camshaft Bent pushrods Worn rocker arms Casting flash in the intake or exhaust manifold passages |
| Additional Checks | Inspect for electromagnetic interference (EMI) by checking for the following: Monitoring engine speed on the scan tool may indicate EMI is present. A sudden increase in scan tool engine speed while actual engine speed remains constant indicates EMI is present. EMI on the ignition control circuit may cause a missing condition. |
| IMPORTANT |
|---|
| A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. |
Poor Fuel Economy
| Checks | Action |
|---|
| DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the current fuel economy is noticeably lower than the vehicle's previous fuel economy, as shown by an actual road test. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . Inspect the air filter for proper type and for dirt or restriction. Inspect the vacuum hoses for leaks, restrictions and proper connections as shown on the vehicle emission control information (VECI) label. Review the vehicle operators driving habits for the following items: The A/C compressor clutch being engaged all the time Excessive idling or stop and go driving Proper tire inflation Low CNG refill pressure - check fill station pressure Excessively heavy loads being carried Too much acceleration, too often Advise vehicle operator to fill the fuel tank and recheck fuel economy. |
| Fuel System Checks | IMPORTANT: The customer may comment that one fuel tank does not appear to fill. An inoperative high pressure lock-off (HPL) solenoid valve will allow a fuel tank to fill, but will not allow it to empty. Test all of the high pressure lock-off (HPL) solenoids for proper operation. Test by performing the following: Disconnect the HPL solenoid power and ground connector at each CNG fuel tank except one. Operate the engine on CNG for at least 2 minutes while only one HPL solenoid is connected. Observe the fuel gauge while the engine is operating. Repeat the above procedure for each HPL until all CNG fuel tanks have independently operated the engine. An HPL condition exists if any of the following symptoms are observed: The fuel gauge drops toward empty. The engine switches over to gasoline operation (KL6). The engine stalls (KL8). If an HPL condition exists, test for the following: A closed HPL manual shut-off valve. A lack of power or ground at the HPL connector when commanded ON. An open or shorted HPL solenoid. Replace the HPL as necessary. Refer to High Pressure Lock-Off (HPL) Solenoid Replacement . Test the fuel pressure. Refer to Fuel System Diagnosis . Inspect for fuel system leakage. |
| Sensor Checks | Test the ECT sensor for proper operation by performing the following: Allow the engine temperature to reach ambient air temperature by leaving the engine OFF for several hours - overnight for most accurate results. Turn the ignition ON while leaving the engine OFF. Using a scan tool compare the ECT and IAT sensors displayed temperature. The sensors should be within 3° C (5° F) of each other. Replace the ECT sensor or repair the ECT sensor circuit if the ECT is not near the expected value. |
| Ignition System Checks | Remove and inspect the spark plugs for the following conditions: Wet plugs Cracks Wear Improper gap Burned electrodes Heavy deposits Test for an open ignition control (IC) circuit. Inspect the spark plug wires for the following: Cracking Hardness Proper connections and routing IMPORTANT: More timing advance is required to operate the vehicle on CNG than on gasoline. Test for proper operation of the Knock Sensor system. |
| IMPORTANT |
|---|
| The customer may comment that one fuel tank does not appear to fill. An inoperative high pressure lock-off (HPL) solenoid valve will allow a fuel tank to fill, but will not allow it to empty. |
| IMPORTANT |
|---|
| More timing advance is required to operate the vehicle on CNG than on gasoline. |
Poor Fuel Fill Quality
| Checks | Action |
|---|
| DEFINITION 1: Fuel filling time, as measured against previous filling time, is noticeably longer than expected. DEFINITION 2: Only one CNG fuel tank appears to fill. |
| Fuel System Checks | Inspect for the following conditions: A contaminated or clogged fill valve filter. A service interval is not specified for the fill valve filter. Its condition depends on fuel quality. Use of CNG with excessive amounts of contaminants or compressor oil will increase the frequency of service. Service if necessary. A fill line union check valve that is restricted or improperly installed. The check valve flow direction arrow should point towards the fuel tank. IMPORTANT: The customer may comment that one fuel tank does not appear to fill. An inoperative high pressure lock-off (HPL) solenoid valve will allow a fuel tank to fill, but will not allow it to empty. Test all of the high pressure lock-off (HPL) solenoids for proper operation. Test by performing the following: Disconnect the HPL solenoid power and ground connector at each CNG fuel tank except one. Operate the engine on CNG for at least 2 minutes while only one HPL solenoid is connected. Observe the fuel gauge while the engine is operating. Repeat the above procedure for each HPL until all CNG fuel tanks have independently operated the engine. An HPL condition exists if any of the following symptoms are observed: The fuel gauge drops toward empty. The engine switches over to gasoline operation (KL6). The engine stalls (KL8). If an HPL condition exists, test for the following: A closed HPL manual shut-off valve. A lack of power or ground at the HPL connector when commanded ON. An open or shorted HPL solenoid. Replace the HPL as necessary. Refer to High Pressure Lock-Off (HPL) Solenoid Replacement . |
| IMPORTANT |
|---|
| The customer may comment that one fuel tank does not appear to fill. An inoperative high pressure lock-off (HPL) solenoid valve will allow a fuel tank to fill, but will not allow it to empty. |
Rough, Unstable, or Incorrect Idle and Stalling
| Checks | Action |
|---|
| DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Sensor Checks | Test for proper operation and performance of the oxygen sensor. Inspect the oxygen sensor for silicon contamination. Caused by fuel or the use of improper engine sealants, the oxygen sensor will have a white, powdery coating. The sensor will output a high, but false signal voltage. This false rich condition may cause the module to reduce the amount of fuel to the engine. Test the ECT sensor for proper operation by performing the following: Allow the engine temperature to reach ambient air temperature by leaving the engine OFF for several hours - overnight for most accurate results. Turn the ignition ON while leaving the engine OFF. Using a scan tool compare the ECT and IAT sensors displayed temperature. The sensors should be within 3° C (5° F) of each other. Replace the ECT sensor or repair the ECT sensor circuit if the ECT is not near the expected value. Test for proper MAP sensor operation. A MAP sensor that does not respond quickly and accurately to manifold pressure changes may cause this symptom. |
| Fuel System Checks | Determine if the condition is rich or lean by performing the following: Operate the vehicle under the conditions of the concern. Monitor the oxygen sensor output. Test for leaking fuel injectors. Inspect for fuel - gasoline - in the fuel pressure regulator vacuum hose, if equipped. This indicates a faulty gasoline fuel pressure regulator. Inspect for proper operation of the cng fuel injectors. Refer to: Fuel Injector Coil Test Fuel Injector Balance Test with Special Tool Fuel Injector Balance Test with Tech 2 Test for proper EVAP Purge system operation. Test for proper alternative fuel system pressures. Refer to Fuel System Diagnosis . |
| Additional Checks | Inspect for manifold or air induction system leaks. Test the PCV system for proper operation and valve type. Test for proper engine cylinder compression. |
Rough, Unstable, or Incorrect Idle and Stalling
Dieseling, Run-On
| Checks | Action |
|---|
| DEFINITION: The engine continues to run when the ignition is turned OFF, but usually runs very roughly. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Fuel System Checks | Test for proper EVAP Purge system operation - KL6 only. Test the fuel injectors for leakage. Inspect for correct throttle blade adjustment, if applicable. Perform the fuel system diagnostic procedure. Refer to Fuel System Diagnosis . |
| Additional Checks | If the engine continues to run smoothly when the ignition is turned OFF inspect for the following: Proper ignition switch adjustment A short to voltage in the modules ignition feed circuit. |
Backfire
| Checks | Action |
|---|
| DEFINITION: The fuel ignites in the intake manifold, or in the exhaust system, making a loud popping noise. |
| Preliminary Checks | Refer to Important Preliminary Checks in Symptoms - Engine Controls . |
| Ignition System Checks | IMPORTANT: A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. Inspect for proper secondary ignition system voltage output using only the J 26792 spark tester. Remove and inspect the spark plugs for the following conditions: Wet plugs Cracks Wear Improper gap Burned electrodes Heavy deposits Test for proper ignition system operation. Test for crossfire between spark plugs. Test for spark plug wire failure. Wetting down the secondary ignition system with a water spray bottle may aid in locating damaged or deteriorated components. |
| Engine Mechanical Checks | Inspect the engine for the following: Intake manifold leaks Improper valve timing Proper engine cylinder compression Sticking or leaking intake or exhaust valves Exhaust system leakage Inspect the intake and exhaust system for casting flash. |
| Fuel System Checks | Perform fuel system diagnosis. Refer to Fuel System Diagnosis . |
| Additional Checks | Review service bulletins for module programming updates. Verify that the correct CNG software and calibration has been programmed into the PCM. Test for proper operation of the AFO Enable circuit. |
| IMPORTANT |
|---|
| A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline. |
Circuit Description
The powertrain control module (PCM) controls the fuel system while the vehicle is operating on CNG. A high pressure lock-off (HPL) solenoid in each CNG tank and a low pressure lock-off (LPL) solenoid in the fuel rail prevent fuel flow. The PCM commands ONLY the HPL's open for 1 second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tanks. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.
The lock-off relay's coil and switch are supplied power whenever the ignition is in the Start or Run position. The PCM controls the ground path. When the lock-off relay's are energized they supply power to the high pressure lock-off (HPL) solenoid and the low pressure lock-off (LPL) solenoid.
| Step | Action | Yes | No |
|---|
| Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views or Fuel Injection Control Module Connector End Views |
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Is DTC P0005 or P2665 set? | Go to DTC P0005 or DTC P2665 | Go to Step 3 |
| 3 | Turn ON the ignition with the engine OFF. Command the high pressure lock-off (HPL) ON and OFF with a scan tool. Listen for operation of the HPLs. Do the HPLs respond to the scan tool commands? | Go to Step 4 | Go to Step 5 |
| 4 | Command the low pressure lock-off (LPL) ON and OFF with a scan tool. Listen for operation of the LPL. Does the LPL respond to the scan tool commands? | Go to Step 18 | Go to Step 6 |
| 5 | Command the HPL solenoids ON and OFF with a scan tool. Listen or feel for operation of the HPL relay. Does the HPL relay respond to the scan tool commands? | Go to Step 14 | Go to Step 15 |
| 6 | Command the LPL solenoid ON and OFF with a scan tool. Listen or feel for operation of the LPL relay. Does the LPL relay respond to the scan tool commands? | Go to Step 8 | Go to Step 7 |
| 7 | Disconnect the LPL relay. Probe the relay switch ignition circuit terminal (30) with a test lamp connected to ground. Turn ON the ignition with the engine OFF. Is the test lamp illuminated? | Go to Step 10 | Go to Step 11 |
| 8 | Disconnect the LPL solenoid. Probe the LPL solenoid ground circuit terminal with a test lamp connected to B+. Is the test lamp illuminated? | Go to Step 9 | Go to Step 13 |
| 9 | Probe the LPL solenoid ignition voltage circuit terminal with a test lamp connected to ground. Command the LPL solenoid ON and OFF with a scan tool. Does the test lamp respond to the scan tool commands? | Go to Step 17 | Go to Step 12 |
| 10 | Test for an intermittent and for a poor connection at the LPL relay. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems in the G/H Van Service Manual. Did you find and correct the condition? | Go to Step 18 | Go to Step 16 |
| 11 | Repair the LPL relay switch power circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair? | Go to Step 18 | |
| 12 | Repair the LPL solenoid power circuit between the relay and the solenoid for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair? | Go to Step 18 | |
| 13 | Repair the LPL solenoid ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair? | Go to Step 18 | |
| 14 | Repair the HPL solenoid's ground circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the G/H Van Service Manual. Did you complete the repair? | Go to Step 18 | |
| 15 | Replace the HPL relay. Did you complete the replacement? | Go to Step 18 | |
| 16 | Replace the LPL relay. Did you complete the replacement? | Go to Step 18 | |
| 17 | Replace the LPL solenoid. Refer to Low Pressure Lock-Off (LPL) Solenoid Replacement . Did you complete the replacement? | Go to Step 18 | |
| 18 | Clear DTC information with a scan tool from the PCM. Turn the ignition OFF and wait 30 seconds. Start and idle the engine. Observe the MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? | System OK | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
Fuel Lock-Off Relay Diagnosis
System Description
The powertrain control module (PCM) controls the fuel system while the vehicle is operating on CNG. The PCM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in cylindrical fuel tanks. A full tank can contain fuel at 24,820 kPa (3600 psi). A high pressure lock-off (HPL) solenoid in each tank and a low pressure lock-off (LPL) solenoid in the fuel line prevent fuel flow. The PCM commands ONLY the HPL's open for 1 second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tanks. The PCM commands BOTH the HPL's and the LPL open when engine RPM indicates the engine is cranking or running on CNG.
This fuel delivery system utilizes a high pressure regulator (HPR) in order to lower the fuel pressure from tank pressure down to an injector delivery pressure.
When the PCM commands the HPL's open, fuel begins to flow through the HPR. The HPR reduces pressure to between 896-1379 kPa (130-200 psi). The fuel exiting the HPR flows through the intermediate fuel line to the fuel rail mounted low pressure lock-off (LPL) solenoid. Fuel flows out of the LPL and through the fuel rails to the injectors.
Test Description
The number below refers to the step number on the diagnostic table.
- 2: This step verifies that the HPL solenoid and the FTP sensor are functioning. A fuel tank pressure value that decreases while the engine is operating indicates the HPL is not opening or the manual tank valve is closed.
- 3: This step verifies the FRP sensor is displaying the correct fuel pressure.
- 4: This step tests for a leak in the fuel system between the low pressure lock-off (LPL), and the fuel injectors. The fuel pressure should remain constant during the test.
- 5: This step tests the HPR output pressure. If the fuel pressure is within the specified values, then the HPR, fuel lines, and filters are OK.
- 6: This step tests the operation of the HPL relay.
- 9: This step tests the operation of the LPL relay.
- 13: This step verifies that a fuel filter is not the cause of the condition. If the fuel pressure is within the specified range after the fuel filter replacement, then a plugged filter was the cause of the condition.
| Step | Action | Value(s) | Yes | No |
|---|
| Schematic Reference: Engine Controls Schematics |
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Connect the vehicle to a CNG dispensing station and verify that the CNG fuel tank is at least 1/3 full. Fill CNG fuel tank if necessary. Observe the fuel tank pressure (FTP) sensor data display with a scan tool. Attempt to start and idle the engine. Does the FTP sensor display a steady pressure at or above the specified value? | 8274 kPa (1200 psi ) | Go to Step 3 | Go to Step 6 |
| 3 | Attempt to start and idle the engine. Idle the engine until normal operating temperature is obtained. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Observe and record the FRP sensor display with a scan tool. Compare the recorded scan tool display pressure values to the value range specified. Is the FRP sensor value within the specified range? | 896-1379 kPa (130-200 psi) | Go to Step 4 | Go to Step 8 |
| 4 | Turn OFF the ignition. Turn ON the ignition with the engine OFF. Command the LPL solenoid ON and OFF with a scan tool in order to charge the fuel rail with pressure. Monitor the fuel rail pressure for 5 minutes after commanding the LPL solenoid closed. Does the fuel rail pressure drop more than the specified value? | 21 kPa (3 psi) | Go to Step 10 | Go to Step 5 |
| 5 | IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Observe the FRP sensor display with the scan tool while cranking the engine. Idle the engine until normal operating temperature is obtained. IMPORTANT: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the FRP sensor display with a scan tool at engine idle. Observe and record the pressure indicated by the FRP sensor display during engine load by performing the following steps: Apply the parking brake. Firmly apply and hold the brake pedal. Place the vehicle in forward gear. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds. Allow the engine to idle for a minimum of two minutes in order to cool the transmission. Compare the recorded pressure values to the value range specified. Are all of the fuel pressure values within the specified range? | 896-1379 kPa (130-200 psi) | Go to Intermittent Conditions | Go to Step 12 |
| 6 | Turn ON the ignition with the engine OFF. Command the HPL ON and OFF with a scan tool. Listen for operation of the HPLs. Can operation of the HPLs be heard? | | Go to Step 7 | Go to Fuel Lock-Off Relay Diagnosis |
| 7 | Ensure the manual tank valves are fully open by turning the screws fully counter clockwise. Did you have to open the manual tank valves? | | Go to Step 2 | Go to Step 14 |
| 8 | Is the fuel pressure greater than the specified value? | 1379 kPa (200 psi) | Go to Step 15 | Go to Step 9 |
| 9 | Turn the ignition ON with the engine OFF. Command the low pressure lock-off (LPL) ON and OFF with a scan tool. Listen for operation of the LPL. Can operation of the LPL be heard? | | Go to Step 12 | Go to Fuel Lock-Off Relay Diagnosis |
| 10 | Inspect for a fuel system leak between the LPL solenoid and the CNG fuel injectors. Replace any leaking components as necessary. Did you find and correct the condition? | | Go to Step 16 | Go to Step 11 |
| 11 | Raise the CNG fuel rails, lines and injectors as an assembly. Refer to Fuel Rail Assembly Replacement (KL6) or Fuel Rail Assembly Replacement (KL8) . Command the LPL ON with a scan tool. Locate and replace the leaking CNG fuel injector. Refer to Fuel Injector Replacement . Did you complete the replacement? | | Go to Step 16 | |
| 12 | Inspect the fuel line between the HPL and the HPR and the fuel line between the HPR and the LPL for the following: Kinks Bends Damage Obstruction An obstructed fuel line may be cool or frosted near the obstruction when fuel flows through the line. Repair or replace components as necessary. Did you find and correct the condition? | | Go to Step 16 | Go to Step 13 |
| 13 | Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief Procedure . Replace the CNG In-line filter. Refer to Fuel Filter Replacement (Passenger/Cargo) or Fuel Filter Replacement (Cutaway) . Replace the HPR filter. Refer to High Pressure Regulator (HPR) Filter Replacement . IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Observe the FRP sensor display with a scan tool while cranking the engine. Idle the engine until normal operating temperature is obtained. IMPORTANT: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the FRP sensor display at engine idle. Observe and record the FRP sensor display during engine load by performing the following steps: Apply the parking brake. Firmly depress and hold the brake pedal. Place the vehicle in forward gear. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds. Allow the engine to idle for a minimum of two minutes in order to cool the transmission. Compare the recorded pressure values to the value range specified. Are all fuel pressure values within the specified range? | 896-1379 kPa (130-200 psi) | Go to Step 16 | Go to Step 15 |
| 14 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement? | | Go to Step 16 | |
| 15 | Replace the HPR. Refer to High Pressure Regulator (HPR) Replacement (Passenger/Cargo) or High Pressure Regulator (HPR) Replacement (Cutaway) . Did you complete the replacement? | | Go to Step 16 | |
| 16 | Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief Procedure . Turn OFF the ignition. Remove all remaining test equipment. Start and idle the engine. Inspect for CNG fuel system leaks with the J 41416 Ultrasonic Leak Detector and Snoop® or soapy water. Repair any leaks as necessary. Clear DTC information with a scan tool from the PCM. Turn the ignition OFF and wait 30 seconds. Start the engine and operate the vehicle. Observe the MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? | | System OK | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| IMPORTANT |
|---|
| If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. |
| IMPORTANT |
|---|
| If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. |
| IMPORTANT |
|---|
| Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. |
| IMPORTANT |
|---|
| If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. |
| IMPORTANT |
|---|
| Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. |
The powertrain control module (PCM) enables the appropriate CNG fuel injector on the intake stroke for each cylinder. The PCM is not capable of operating the high current compressed natural gas (CNG) fuel injectors. The PCM injector pulse width signals are received by the fuel injector control module (FICM) and duplicate pulse width signals are generated by the FICM in order to operate the CNG injectors. The FICM controls each fuel injector by grounding the control circuit via a solid state device called a driver. A fuel injector coil winding resistance that is too high, or low, will affect engine driveability. A fuel injector control circuit DTC may not set, but a misfire may be apparent. The fuel injector coil windings are affected by temperature. The resistance of the fuel injector coil windings will increase as the temperature of the fuel injector increases.
Diagnostic Aids
- Operating the vehicle over a wide temperature range may help isolate the fuel injector that is causing the condition.
- Perform the fuel injector coil test within the conditions of the customers concern. A fuel injector condition may only be apparent at a certain temperature, or under certain conditions.
- If the fuel injector coil test does not isolate the condition perform the fuel injector balance test. Refer to «Fuel Injector Balance Test with Special Tool»(/chevrolet/chevy-express-g1500/1996-2012/remont/testing-diagnostics/#engine-control-system-60l-alternative-fuel-troubleshooting) or «Fuel Injector Balance Test with Tech 2»(/chevrolet/chevy-express-g1500/1996-2012/remont/testing-diagnostics/#engine-control-system-60l-alternative-fuel-troubleshooting) .
The numbers below refer to the step numbers on the diagnostic table.
- 3: This step tests each fuel injector resistance within a specific temperature range. If any of the fuel injectors display a resistance outside of the specified value, replace the fuel injector.
- 4: This step determines if all of the fuel injectors are within 0.5 ohms of each other. If the highest resistance value is within 0.5 ohms of the lowest resistance value, then all of the fuel injector coil windings are OK.
- 5: This step determines which fuel injector is faulty. After subtracting the highest and lowest resistance values from the average value, replace the fuel injector that has the greatest resistance difference from the average.
| Step | Action | Values | Yes | No |
|---|
| Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Controls Connector End Views |
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Observe the engine coolant temperature (ECT) with a scan tool. Is the ECT value within the specified range? | 10-32° C (50-90° F) | Go to Step 3 | Go to Step 4 |
| 3 | NOTE: Use care in removing the fuel injectors in order to prevent damage to the fuel injector electrical connector pins or the fuel injector nozzles. Do not immerse the fuel injector in any type of cleaner. The fuel injector is an electrical component and may be damaged by this cleaning method. IMPORTANT: Calibrate the DMM and test leads to 0 ohms in order to prevent misdiagnosis. Measure the resistance of each compressed natural gas (CNG) fuel injector with a DMM. Refer to Testing for Continuity in Wiring Systems in the G/H Van Service Manual.Do any of the fuel injectors display a resistance outside the specified range? | 4.56-5.04 ohms | Go to Step 6 | Go to Diagnostic Aids |
| 4 | NOTE: Use care in removing the fuel injectors in order to prevent damage to the fuel injector electrical connector pins or the fuel injector nozzles. Do not immerse the fuel injector in any type of cleaner. The fuel injector is an electrical component and may be damaged by this cleaning method. Measure the resistance of each CNG fuel injector with a DMM. Refer to Testing for Continuity in Wiring Systems in the G/H Van Service Manual. Record each fuel injector value. Subtract the lowest resistance value from the highest resistance value. Is the difference equal to, or less than, the specified value? | 0.5 ohms | Go to Diagnostic Aids | Go to Step 5 |
| 5 | Add all of the CNG fuel injector resistance values, to obtain a total resistance value. Divide the total resistance value by the number of fuel injectors, to obtain an average resistance value. Subtract the lowest, and the highest, individual fuel injector resistance values from the average resistance value. Replace the CNG fuel injector that displays the greatest resistance difference, above or below the average. Refer to Fuel Injector Replacement . Did you complete the replacement? | | Go to Step 7 | |
| 6 | Replace the fuel injector or fuel injectors that are out of the specified range. Refer to Fuel Injector Replacement . Did you complete the replacement? | | Go to Step 7 | |
| 7 | Operate the system in order to verify the repair. Did you correct the condition? | | System OK | Go to Step 2 |
| NOTE |
|---|
| Use care in removing the fuel injectors in order to prevent damage to the fuel injector electrical connector pins or the fuel injector nozzles. Do not immerse the fuel injector in any type of cleaner. The fuel injector is an electrical component and may be damaged by this cleaning method. |
| IMPORTANT |
|---|
| Calibrate the DMM and test leads to 0 ohms in order to prevent misdiagnosis. |
| NOTE |
|---|
| Use care in removing the fuel injectors in order to prevent damage to the fuel injector electrical connector pins or the fuel injector nozzles. Do not immerse the fuel injector in any type of cleaner. The fuel injector is an electrical component and may be damaged by this cleaning method. |
The J 39021 Fuel Injector Tester energizes the CNG fuel injector for a precise amount of time allowing a measured amount of fuel into the manifold. This causes a drop in system fuel pressure that can be recorded and used to compare each fuel injector.
The numbers below refer to the step numbers on the diagnostic table.
- 3: The fuel pressure should be within the specified range. Refer to Fuel System Diagnosis if the fuel pressure is not within the specified range.
- 5: If the pressure drop value for each fuel injector is within 14 kPa (2.0 psi) of the average pressure drop value, the fuel injectors are flowing properly. Calculate the pressure drop value for each fuel injector by subtracting the second pressure reading from the first pressure reading.
| Step | Action | Values | Yes | No |
|---|
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Did you perform the Fuel Injector Coil Test? | | Go to Step 3 | Go to Fuel Injector Coil Test |
| 3 | Start and idle the engine. Observe the fuel rail pressure sensor parameter with a scan tool. Is the fuel pressure within the specified range? | 896-1379 kPa (130-200 psi) | Go to Step 4 | Go to Fuel System Diagnosis |
| 4 | Turn OFF the ignition. Turn ON the ignition, with the engine OFF. Monitor the fuel rail pressure sensor parameter with a scan tool. Does the fuel rail pressure remain constant? | | Go to Step 5 | Go to Fuel System Diagnosis |
| 5 | NOTE: Do Not repeat any portion of this test before running the engine in order to prevent the engine from flooding. Turn OFF the ignition. Connect the J 39021 Fuel Injector Tester to a fuel injector. Set the amperage supply selector switch on the fuel injector tester to the Balance Test 0.5-2.5 amp position. Turn ON the ignition, with the engine OFF. Record the fuel pressure indicated by the fuel rail pressure sensor after the fuel pressure stabilizes. This is the first pressure reading. IMPORTANT: Record the fuel pressure value immediately after the CNG fuel injector stops pulsing. The fuel pressure may rise after the fuel injector stops pulsing. Do not record the higher fuel pressure value. Energize the CNG fuel injector by depressing the Push to Start Test button on the fuel injector tester. Record the fuel pressure indicated by the fuel rail pressure sensor. This is the second fuel pressure reading. Repeat steps 1 through 6 for each fuel injector. Subtract the second pressure reading from the first pressure reading for one fuel injector. The result is the pressure drop value. Obtain a pressure drop value for each fuel injector. Add all of the individual pressure drop values. This is the total pressure drop. Divide the total pressure drop by the number of fuel injectors. This is the average pressure drop. Does any fuel injector have a pressure drop value that is more than the average pressure drop or less than the average pressure drop by the specified value? | 14 kPa (2.0 psi) | Go to Step 6 | Go to Symptoms - Engine Controls |
| 6 | Replace the affected CNG fuel injector. Refer to Fuel Injector Replacement . Did you complete the replacement? | | Go to Step 7 | |
| 7 | Operate the vehicle in order to verify the repair. Does a driveability condition still exist? | | Go to Symptoms - Engine Controls | System OK |
| NOTE |
|---|
| Do Not repeat any portion of this test before running the engine in order to prevent the engine from flooding. |
| IMPORTANT |
|---|
| Record the fuel pressure value immediately after the CNG fuel injector stops pulsing. The fuel pressure may rise after the fuel injector stops pulsing. Do not record the higher fuel pressure value. |
Fuel Injector Balance Test Procedure
The scan tool first energizes the following components for a precise amount of time allowing a measured amount of fuel into the manifold
- High pressure lock off (HPL)
- Low pressure lock off (LPL)
- CNG fuel injectors
This causes a drop in system fuel pressure that can be recorded and used to compare each fuel injector.
The numbers below refer to the step numbers on the diagnostic table.
- 3: The fuel pressure should be within the specified range. Refer to Fuel System Diagnosis if the fuel pressure is not within the specified range.
- 4: The fuel pressure should reach a steady value. Refer to Fuel System Diagnosis if the fuel pressure does not stabilize.
- 5: If the pressure drop value for each fuel injector is within 14 kPa (2.0 psi) of the average pressure drop value, the fuel injectors are flowing properly. Calculate the pressure drop value for each fuel injector by subtracting the second pressure reading from the first pressure reading.
| Step | Action | Values | Yes | No |
|---|
| 1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
| 2 | Did you perform the Fuel Injector Coil Test? | | Go to Step 3 | Go to Fuel Injector Coil Test |
| 3 | Start and idle the engine. Observe the fuel rail pressure sensor parameter with a scan tool. Is the fuel pressure within the specified range? | 896-1379 kPa (130-200 psi | Go to Step 4 | Go to Fuel System Diagnosis |
| 4 | Turn OFF the ignition. Turn ON the ignition, with the engine OFF. Monitor the fuel rail pressure sensor parameter with a scan tool. Does the fuel rail pressure remain constant? | | Go to Step 5 | Go to Fuel System Diagnosis |
| 5 | NOTE: Do Not repeat any portion of this test before running the engine in order to prevent the engine from flooding. With a scan tool, select the Fuel Injector Balance Test function, within the Special Functions menu. Select an injector to be tested. Press Enter. This will prime the fuel system. Record the fuel pressure indicated by the fuel pressure sensor after the fuel rail pressure stabilizes. This is the 1st pressure reading. IMPORTANT: Record the fuel pressure value immediately after the fuel injector stops pulsing. The fuel pressure may rise after the fuel injector stops pulsing. Do not record the higher fuel pressure value. Energize the fuel injector by depressing the Pulse Injector button on the scan tool. This will energize the injector and decrease the fuel pressure. Record the fuel pressure indicated by the fuel rail pressure sensor after the fuel injector has stopped pulsing. This is the 2nd pressure reading. Press Enter again to bring you back to the Select Injector screen. Repeat for each fuel injector. Subtract the 2nd pressure reading from the 1st pressure reading for one fuel injector. The result is the pressure drop value. Obtain a pressure drop value for each fuel injector. Add all of the individual pressure drop values. This is the total pressure drop. Divide the total pressure drop by the number of fuel injectors. This is the average pressure drop. Does any fuel injector have a pressure drop value that is either higher than the average pressure drop or lower than the average pressure drop by the specified value? | 14 kPa (2.0 psi) | Go to Step 6 | Go to Symptoms - Engine Controls |
| 6 | Replace the affected fuel injector. Refer to Fuel Injector Replacement . Did you complete the replacement? | | Go to Step 7 | |
| 7 | Operate the vehicle in order to verify the repair. Does a driveability condition still exist? | | Go to Symptoms - Engine Controls | System OK |
| NOTE |
|---|
| Do Not repeat any portion of this test before running the engine in order to prevent the engine from flooding. |
| IMPORTANT |
|---|
| Record the fuel pressure value immediately after the fuel injector stops pulsing. The fuel pressure may rise after the fuel injector stops pulsing. Do not record the higher fuel pressure value. |
Fuel Injector Balance Test Procedure
