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 HPR or in-line filters. Refer to High Pressure Regulator (HPR) Filter Replacement and Fuel Filter Replacement . |
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 high pressure regulator (HPR) filter for restriction. Refer to High Pressure Regulator (HPR) Filter Replacement . |
| 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 System Diagnosis . 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 module 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 compressed natural gas (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 | Test the fuel pressure. Refer to Fuel System Diagnosis . Inspect for fuel system leakage. |
| Sensor Checks | Test the engine coolant temperature (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 intake air temperature (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 |
|---|
| More timing advance is required to operate the vehicle on CNG than on gasoline. |
Poor Fuel Fill Quality
| Checks | Action |
|---|
| DEFINITION: Fuel filling time, as measured against previous filling time, is noticeably longer than expected. |
| 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. |
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 PCM 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 PCM 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 PCM 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. |
High Pressure Regulator Stage
When the PCM commands the HPL open fuel begins to flow through the high pressure regulator (HPR). The HPR reduces fuel pressure to between 758-1,448 kPa (110-210 psi). The outlet of the HPR is the intermediate pressure stage.
Intermediate Pressure Regulator Stage
With a pressure between 758-1,448 kPa (110-210 psi) the fuel exiting the HPR flows through the intermediate fuel line to the LPL solenoid. The LPL is controlled by the PCM. Fuel flows out of the LPL and into the intermediate pressure regulator (IPR). The IPR reduces fuel pressure to between 246-317 kPa (43-46 psi). Fuel flows out of the IPR and into the fuel rails.
