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Engine Controls - Trouble Shooting - No Codes - 6.0L W/bifuel & Cng Chevrolet Chevy Express H2500

Testing & Diagnostics ~4466 words

INTRODUCTION

Note. Unless indicated otherwise, Savana includes Savana Special.

Before diagnosing symptoms or intermittent faults, follow proper workflow. See DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article or DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS - ENGINE CONTROLS in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article. If no Diagnostic Trouble Codes (DTC) are present and a no start condition exists, proceed to NO-START DIAGNOSIS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article. If no DTCs are present and a driveability condition exists, diagnosis by symptom (i.e., ROUGH IDLE, ENGINE STALLS, etc.).

Note. Some driveability problems may have been corrected by manufacturer with a revised computer calibration chip or computer control unit. Check with manufacturer for latest chip or computer application.

Use intermittent test procedures to locate driveability problems that do not occur when the vehicle is being tested. These test procedures should also be used if a soft (intermittent) trouble code was present but no problem was found during self-diagnostic testing.

PRELIMINARY INSPECTION

  1. Verify on-board diagnostics are working by performing «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS - ENGINE CONTROLS»(ref-159297-S00366181602003112400000) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article or «DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS - ENGINE CONTROLS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-system-check-alternative-fuels) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article.
  2. Ensure PCM and MIL functions properly.
  3. Ensure no DTCs (except intermittent) are stored.
  4. Perform fuel system pressure test. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  5. Visually inspect all systems.

After all checks have been performed, verify customer complaint and locate correct symptom. Check items indicated under that symptom. See SYMPTOMS .

SYMPTOMS

Note. Before proceeding with any symptom diagnosis, perform PRELIMINARY INSPECTION .

Note. If PCM displays data but engine fails to start, see NO-START DIAGNOSIS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

SYMPTOM DIAGNOSIS

Symptom checks should not be used unless symptom occurs while vehicle is being tested. To reduce diagnostic time, ensure steps in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG and SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG or SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG articles were performed before diagnosing a symptom. Symptoms available for diagnosis include

  1. «ENGINE BACKFIRES»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  2. «ENGINE CUTS OUT, MISSES»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  3. «ENGINE DIESELING/RUN-ON»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  4. «HARD START»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  5. «HESITATION, SAG OR STUMBLE»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  6. «LACK OF POWER OR SLUGGISH»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  7. «POOR FUEL FILL QUALITY»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  8. «POOR FUEL ECONOMY»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  9. «ROUGH, UNSTABLE OR INCORRECT IDLE & STALLING»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)
  10. «VEHICLE SURGES OR CHUGGLES»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng)

Symptom Definition

Fuel ignites within intake manifold or in exhaust system, making a loud popping noise.

Possible Causes & Corrections

Check the following items

  1. Inspect for proper secondary ignition system voltage output using Spark Tester (J-26792).
  2. Remove and inspect spark plugs for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.
  3. Test for proper ignition system operation.
  4. 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.
  5. Inspect engine for intake manifold leaks, improper valve timing, proper engine cylinder compression, sticking or leaking intake or exhaust valves or exhaust system leakage.
  6. Inspect intake and exhaust system for casting flash.
  7. Check fuel system performance. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  8. Review service bulletins for AF ECM or PCM programming updates.
  9. Verify that correct CNG software and calibration has been programmed into PCM.

Cuts out, misses is defined as a steady pulsation or jerking following engine speed, usually more pronounced as engine load increases. Exhaust may have a steady spitting sound at idle or low speed. Perform VISUAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

Check the following items

  1. Check fuel pressure. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  2. Inspect for proper operation of CNG fuel injectors. See «FUEL SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-60l-wbifuel-cng) in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  3. Test the high and low pressure lock-off valves/circuits for proper operation.
  4. Check for proper secondary ignition system voltage output using Spark Tester (J-26792).
  5. Inspect for fouled spark plugs. If fouled spark plugs are indicated, repair cause of fouling.
  6. Test for faulty spark plug wires.
  7. Test for an open ignition system ground, if equipped.
  8. Test for proper engine operation by checking for correct engine compression, correct engine valve timing, worn or incorrect camshaft, bent push rods, worn rocker arms, or casting flash in intake or exhaust manifold passages.
  9. Monitoring engine speed on 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.
  10. EMI on ignition control circuit may cause a missing condition.

Engine continues to run (but very rough) after ignition is turned off. If engine runs smoothly, check ignition switch.

Check for the following

  1. Test for proper EVAP Purge system operation (RPO KL6 engine only). See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  2. Inspect fuel injectors for leakage or diagnose fuel system. See «FUEL SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-60l-wbifuel-cng) in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  3. If engine continues to run smoothly when ignition is turned off, inspect for proper ignition switch adjustment or a short to voltage in the AF ECM or PCM ignition feed circuit.

Engine cranks okay, but does not start immediately. Engine eventually starts and may run okay or soon stall.

Check the following items

  1. Ensure vehicle operator is using correct starting procedure.
  2. Check fuel pressure. See FUEL SYSTEM PRESSURE TEST under «FUEL SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  3. Test for an accurate fuel indication from Fuel Tank Pressure (FTP) sensor.
  4. Test ECT sensor for proper operation by performing the following: Allow engine temperature to reach ambient air temperature by leaving the engine off for several hours or overnight for most accurate results. Turn ignition on, with engine off. Using scan tool, compare ECT and IAT sensors displayed temperature. The sensors should be within 5°F (3°C) of each other. Replace ECT sensor or repair ECT sensor circuit if ECT is not near the expected value.
  5. Test TP sensor for proper operation. With throttle blade at rest position, the monitored TP sensor voltage on a scan tool should be LESS than 0.85 volt. If TP sensor is not near the expected voltage check for a faulty TP sensor, faulty TP sensor circuit, a sticking throttle shaft or linkage/cable assembly.
  6. Test for proper operation of Crankshaft Position (CKP) sensor. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  7. Test for proper operation of MAP sensor, if equipped. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.

Momentary lack of response when accelerator is pushed down. May occur at any vehicle speed. Usually occurs after a stop. If severe enough, may cause engine to stall.

Check for the following items

  1. Check fuel pressure. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article. Inspect for low pressure during moderate or full throttle acceleration. If fuel pressure drops less than specification, there is possibly a low or high pressure regulator fault or a fuel system restriction.
  2. Test for proper EVAP purge system operation. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  3. Test for proper TP sensor operation. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article. A TP sensor that does not respond quickly and accurately to throttle plate movement may cause this symptom.
  4. Test for proper MAP sensor operation. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  5. Test for proper secondary ignition system voltage output using Spark Tester (J-26792).
  6. Inspect for fouled spark plugs. If fouled spark plugs are fouled, repair cause of fouling.
  7. Test for faulty spark plug wires.
  8. Test for an open ignition system ground, if equipped.
  9. Review service bulletins for AF ECM or PCM programming updates.
  10. Inspect for manifold or air induction system leaks.
  11. Test for proper generator operation.

Engine delivers less power than expected. Little or no increase in speed when accelerator is applied.

Check the following items

  1. Compare customer vehicle with a similar unit. Verify the customer's concern.
  2. Remove air filter and check for dirt or a restriction.
  3. Inspect transmission shift pattern and downshift operation.
  4. Inspect PCM for correct CNG calibration. Program PCM, if necessary.
  5. Inspect for proper fuel pressure. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  6. Inspect the High Pressure Regulator (HPR) filter for restriction.
  7. Test for proper secondary ignition system voltage output using Spark Tester (J-26792).
  8. Test for proper operation of the Knock Sensor (KS) system. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  9. Inspect exhaust system for a possible restriction. Restrictions can include a failed catalytic converter, damaged or collapsed pipes, or an internally failed muffler.
  10. Correct engine compression.
  11. Correct engine valve timing.
  12. A worn or incorrect camshaft.
  13. Test for proper A/C compressor clutch operation. An A/C clutch that is engaged all the time may cause this symptom.
  14. Test for proper transmission Torque Converter Clutch (TCC) operation, if equipped. See appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS.
  15. Test for proper generator operation.
  16. Inspect for proper operation of accelerator pedal. A stretched accelerator cable may cause this symptom.

Fuel filling time, as measured against previous filling time, is noticeably longer than expected.

Check the following items

  1. A contaminated or clogged fill valve filter. A service interval is not specified for the fill valve filter. It's 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.
  2. A fill line union check valve that is restricted or improperly installed. The check valve flow direction arrow should point towards the fuel tank.

Fuel economy, as measured by an actual road test, is noticeably lower than expected. Current fuel economy is noticeably lower than previous fuel economy.

Check the following items

  1. Test all of the High Pressure Lock-Off (HPL) solenoids for proper operation. An inoperative HPL will cause a decrease in amount of fuel available. This will be perceived as poor fuel economy.
  2. Inspect air filter for proper type and for dirt or restriction.
  3. Inspect vacuum hoses for leaks, restrictions and proper connections as shown on the Vehicle Emission Control Information (VECI) label.
  4. Review driver's driving habits. Check for 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, or too much acceleration, too often.
  5. Advise vehicle operator to fill the fuel tank and recheck fuel economy.
  6. Test fuel pressure. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  7. Inspect for fuel system leakage.
  8. Test the ECT sensor for proper operation by performing the following: Allow engine temperature to reach ambient air temperature by leaving the engine off for several hours or overnight for most accurate results. Turn ignition on, with engine off. Using scan tool, compare ECT and IAT sensors displayed temperature. The sensors should be within 5°F (3°C) of each other. Replace ECT sensor or repair ECT sensor circuit, if ECT is not near the expected value.
  9. Remove and inspect spark plugs for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.
  10. Test for an open Ignition Control (IC) circuit.
  11. Inspect spark plug wires for cracking, hardness or proper connections and routing.
  12. Test for proper operation of Knock Sensor (KS) system. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.

Engine runs unevenly at idle. If bad enough, vehicle will shake. Idle RPM may vary. Engine idles at incorrect RPM.

Check the following items

  1. Test for proper operation and performance of oxygen sensor. Inspect oxygen sensor for silicone contamination caused by fuel or 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 AF ECM or PCM to reduce amount of fuel to engine.
  2. Test the ECT sensor for proper operation by performing the following: Allow the engine temperature to reach ambient air temperature by leaving engine off for several hours or overnight for most accurate results. Turn ignition on, with engine off. Using scan tool, compare the ECT and IAT sensors displayed temperature. The sensors should be within 5°F (3°C) of each other. Replace ECT sensor or repair ECT sensor circuit, if ECT is not near the expected value.
  3. Test for proper MAP sensor operation. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article. A MAP sensor that does not respond quickly and accurately to manifold pressure changes may cause this symptom.
  4. Determine if condition is rich or lean by operating vehicle under the conditions of the concern. Monitor oxygen sensor output.
  5. Test for leaking fuel injectors.
  6. Inspect for gasoline in the fuel pressure regulator vacuum hose, if equipped. This indicates a faulty gasoline fuel pressure regulator.
  7. Inspect for proper operation of CNG fuel injectors. See «FUEL SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-60l-wbifuel-cng) in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  8. Test for proper EVAP Purge system operation. See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  9. Test for proper alternative fuel system pressures. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  10. Inspect for manifold or air induction system leaks.
  11. Test PCV system for proper operation and valve type. See «EMISSION SYSTEMS & SUB-SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-48l-53l-60l-flex-fuel-gasoline__emission-systems-sub-systems) in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  12. Test for proper engine cylinder compression. See CYLINDER COMPRESSION PRESSURE under «MECHANICAL INSPECTION»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) in PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

Engine power varies under steady throttle or cruise. Feels like vehicle speeds up and slows down even though accelerator pedal position remains constant.

Check for the following items

  1. Ensure driver understands the torque converter clutch operation and A/C compressor operation.
  2. Test for proper operation of the Vehicle Speed Sensor (VSS). See «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng__diagnostic-trouble-code-definitions) in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article «DIAGNOSTIC TROUBLE CODE DEFINITIONS»(ref-159297-S15239216512003112400000) in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
  3. Test for proper operation and performance of the oxygen sensor.
  4. 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.
  5. Check fuel pressure. See «FUEL SYSTEM PRESSURE TEST»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-60l-wbifuel-cng-engines) under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  6. Inspect for proper operation of the CNG fuel injectors. See «FUEL SYSTEMS»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-system-component-testing-48l-53l-60l-flex-fuel-gasoline) in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  7. Ensure that manual shut-off valve is fully open.
  8. Inspect for restricted HPR or in-line filters.

RESTRICTED EXHAUST

For dual exhaust systems a quick check of exhaust flow will help determine which side of the exhaust system is restricted. The side that has less exhaust flow is the side that will be suspect, and diagnosis should begin there. To diagnose exhaust system, refer to EXHAUST SYSTEM BACKPRESSURE under MECHANICAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.

INTERMITTENTS

Note. Intermittent is a DTC or symptom, with a condition that cannot be duplicated.

PRELIMINARY

Perform the Diagnostic System Check - Engine Controls before starting. See DIAGNOSTIC SYSTEM CHECK -- ALTERNATIVE FUELS - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG or SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.

HARNESS OR CONNECTOR INTERMITTENTS

Many intermittent open or shorted circuits come and go with harness or connector movement caused by the following type conditions

  1. Vibration.
  2. Engine torque.
  3. Bumps or rough pavement.

Test for intermittents by performing the applicable procedure from the following list

  1. Move related connectors and wiring while monitoring the appropriate scan tool data.
  2. Move related connectors and wiring with the component commanded on and off, with scan tool. Observe the component operation.
  3. With the engine running, move related connectors and wiring while monitoring engine operation.

Verify whether the harness or connector movement affects any of the following systems

  1. Data displayed.
  2. Component or system operation.
  3. Engine operation.

Repair components as necessary.

ELECTRICAL CONNECTIONS OR WIRING

  1. Intermittents are usually caused by one or more of the following conditions: Poor electrical connections. Terminal tension. Wiring problems.
  2. Carefully inspect the suspected circuit for the following conditions: Poor mating of the connector halves. Terminals backed out or not fully seated in the connector body. Improperly formed or damaged terminals. Test for poor terminal tension. Poor terminal-to-wire connections including terminals crimped over insulation. This requires removing the terminal from the connector body. Corrosion or water intrusion. Pierced or damaged insulation can allow moisture to enter the wiring. The conductor can corrode inside the insulation, with little visible evidence. Look for swollen and stiff sections of wire in the suspect circuits. Wires that are broken inside the insulation. Pinched, cut or rubbed-through wiring in harness. Wiring that is in contact with hot exhaust components.
  3. Repair the condition as necessary.

CONTROL MODULE POWER & GROUNDS, & COMPONENT POWER & GROUNDS

Poor power or ground connections can cause widely varying symptoms.

  1. Test all control module power circuits. Many vehicles have multiple circuits supplying power to the control module. Other components in the system may have separate power circuits that may also need to be tested. Inspect connections at the module or component connectors, fuses, and any intermediate connections between the power source and the module or component. A test light or a Digital Volt-Ohmmeter (DVOM) may indicate that voltage is present, but neither tests the ability of a circuit to carry sufficient current. Ensure that the circuit can carry the current necessary to operate the component. See appropriate POWER DISTRIBUTION in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.
  2. Test all control module ground and system ground circuits. The control module may have multiple ground circuits. Other components in the system may have separate grounds that may also need to be tested. Inspect grounds for clean and tight connections at the grounding point. Inspect the connections at the component and in splice packs, where applicable. Ensure that the circuit can carry the current necessary to operate the component. See appropriate GROUND DISTRIBUTION in SYSTEM WIRING DIAGRAMS article in ELECTRICAL.

TEMPERATURE SENSITIVITY

  1. An intermittent condition may occur when a component or connection reaches normal operating temperature. The condition may occur only when the component or connection is cold, or only when the connection is hot.
  2. The following data may help to diagnose this type of intermittent condition: Freeze Frame/Failure Records. Scan tool snapshot. Vehicle data recorder.
  3. If the intermittent is related to heat, review the following data: High ambient temperatures. Underhood or engine generated heat. Circuit generated heat due to a poor connection, or high electrical load. Higher than normal load conditions, such as towing.
  4. If the intermittent is related to cold, review the following data: Low ambient temperatures. In extremely low temperatures, ice may form in a connection or component. Test for water intrusion. The condition only occurs on a cold start. The condition is not present after the vehicle warms up.
  5. Information from the customer may help to determine if the trouble follows a pattern that is temperature related.

ELECTROMAGNETIC INTERFERENCE & ELECTRICAL NOISE

Some electrical components or circuits are sensitive to Electromagnetic Interference (EMI) or other types of electrical noise. Perform the following procedures

  1. Inspect for a misrouted harness that is too close to a high voltage or high current device. This condition may induce electrical noise on a circuit that could interfere with normal circuit operation. Inspect for wires that are too close to the following devices: Secondary ignition components. Motors. The generator.
  2. Determine whether the electrical system interference is caused by a malfunctioning relay, PCM driven solenoid or switch. These components may cause a sharp electrical surge. Normally, the problem will occur when the malfunctioning component is operating.
  3. Determine whether non-factory or aftermarket add-on accessories are installed in the vehicle. These accessories may lead to an emission related OBD-II failure. Determine if any of the following non-factory or aftermarket add-on accessories is causing the intermittent: Lights. 2-way radios. Amplifiers. Electric motors. Remote starters. Alarm systems. Cellular phones.
  4. Test for an open diode across the A/C compressor clutch and for other open diodes. Some relays may contain a clamping diode.
  5. Test for proper performance of the generator. See ON-VEHICLE TESTING in appropriate GENERATORS & REGULATORS article in STARTING & CHARGING SYSTEMS.
  6. If a DTC is determined to be intermittent and the tests do not reveal a problem, see «VEHICLE DATA RECORDER»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-trouble-shooting-no-codes-60l-wbifuel-cng__vehicle-data-recorder) .

INCORRECT PCM PROGRAMMING

  1. There are only a few situations when reprogramming a PCM is appropriate, such as the following: A new service PCM is installed. A PCM from another vehicle is installed. Revised software or calibration files have been released for this vehicle.
  2. Verify that the PCM contains the correct software or calibration. If incorrect programming is found, reprogram the PCM with the most current software or calibration. See POWERTRAIN CONTROL MODULE under PROGRAMMING in «SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG»(/chevrolet/chevy-express-h2500/2003-2003/remont/testing-diagnostics/#engine-controls-self-diagnostics-60l-bi-fuel-cng) or «SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG»(ref-159297) article.

DUPLICATING FAILURE CONDITIONS

  1. If none of the previous tests are successful, attempt to duplicate or capture the failure conditions.
  2. Freeze Frame/Failure Records data, where applicable, contains the conditions that were present when the DTC set. Perform the following procedure: Review and record Freeze Frame/Failure Records data. Clear DTCs using scan tool. Turn ignition off and wait 15 seconds. Operate vehicle under the same conditions that were noted in Freeze Frame/Failure Records data, as closely as possible. The vehicle must also be operating within the Conditions for Running DTC. Monitor DTC Status for the DTC being tested. The scan tool will indicate Ran, when the enabling conditions have been satisfied long enough for the DTC to run. The scan tool will also indicate whether the DTC passed or failed.
  3. An alternate method is to drive vehicle with DVOM connected to a suspected circuit. An abnormal reading on the DVOM when problem occurs, may help you locate the problem.

SCAN TOOL SNAPSHOT

The scan tool can be set up to take a snapshot of the parameters available via serial data. The snapshot function records live data over a period of time. The recorded data can be played back and analyzed. The scan tool can also graph parameters singly or in combinations of parameters for comparison. The snapshot can be triggered manually at the time the symptom is noticed, or set up in advance to trigger when a DTC sets. An abnormal value captured in the recorded data may point to a system or component that needs to be investigated further. Refer to scan tool user instructions for more information on the Snapshot function.

VEHICLE DATA RECORDER

The Vehicle Data Recorder (J-42598) is connected to the Data Link Connector (DLC) and sent with customer. The vehicle data recorder captures data for later retrieval and analysis by the technician. Refer to vehicle data recorder user instructions for more information.

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