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Home >> Saturn >> 2002 >> SC2 Automatic, MP6 >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Testing & Engine Component Testing >> Fuel Systems (2.2L - Cavalier CNG) >> Fuel System Unwanted Switch To Gasoline >> Diagnostic Procedure
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Diagnostic Procedure

  1. If alternative fuels Diagnostic System Check - Engine Controls has been performed, go to next step. If alternative fuels Diagnostic System Check - Engine Controls has not been performed, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. After performing alternative fuels engine controls diagnostic system check, go to next step.
  2. Using scan tool, communicate with AF ECM and observe AFO SIGNAL and AFO SIGNAL COMMAND parameters. Start engine while monitoring parameters. If both parameters indicate NATURAL GAS OPERATION is enabled, go to next step. If both parameters do not indicate NATURAL GAS OPERATION is enabled, go to step  5.
  3. Monitor Fuel Indicator Light (FIL) located in fuel gauge select switch. If FIL is illuminated, go to MIL ON STEADY under SYSTEM TESTS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. If FIL is not illuminated, go to next step.
  4. Fault is not present at this time. If any additional DTCs are set and need to be repaired, go to appropriate diagnostic procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. If no additional DTCs are set, go to DIAGNOSTIC AIDS .
  5. Using scan tool, communicate with AF ECM and observe fuel pressure sensor parameter. If fuel pressure sensor is more than 1200 psi (8274 kPa), go to next step. If fuel pressure sensor is less than 1200 psi (8274 kPa), go to step  10.
  6. Using scan tool, observe AFO SIGNAL parameter and command AFO ENABLE circuit to natural gas operation. If parameter indicates NATURAL GAS OPERATION is enabled, go to next step. If parameter does not indicate NATURAL GAS OPERATION is enabled, go to step  13.
  7. Natural gas operation will be disabled if any of the following conditions occur:
    • Fuel pressure sensor indicates fuel pressure drops to less than 0.85 volt (350 psi at 70°F or 2412 kPa at 21°C). Natural gas operation will NOT be enabled until fuel pressure sensor indicates fuel pressure is more than 1.50 volts (1000 psi at 70°F or 6890 kPa at 21°C).
    • An AF ECM DTC is set. Not all AF ECM DTCs cause a switch-over to gasoline.
    • A wide-open Mixture Control Valve (MCV) is detected. Check MCV control circuits for proper terminal contact and circuit continuity between MCV and GMS connectors.
    • Engine crank time is more than 8 seconds.
    • The following conditions cause a start-up on gasoline followed by switch-over to natural gas during a vehicle decel condition: An engine coolant temperature of less than 10°F (-12°C) is detected at start-up or every 100th engine start.

    Test FTP sensor, Engine Coolant Temperature (ECT) sensor and MCV for proper operation. Check FPS accuracy by connecting vehicle to a CNG filling station and verifying tank pressure. Repair as necessary. After repairs, go to step  22. If no problems are found, go to next step.

    NOTE: A higher secondary voltage is required for CNG. A vehicle that operates normally on gasoline does not indicate a properly operating ignition system. If vehicle starts and stalls, check for malfunctioning Crankshaft Position (CKP) sensor.
  8. Check ignition system voltage using ONLY Spark Tester (J 26792). Check CKP sensor for proper parameters using scan tool. Remove spark plugs and check for fouled (wet), cracked, excessively worn, incorrectly gapped, burned electrodes or heavy deposits. Check for damaged spark plug wires. While engine is running, spray plug wires with a fine mist of water and note any crossfire or misfire. Check AF ECM and PCM grounds for proper contact. Repair as necessary. After repairs, go to step  22. If no problems are found, go to next step.
  9. Perform fuel system diagnosis. See FUEL SYSTEM DIAGNOSIS . Repair as necessary. After repairs, go to step  22. If fuel system is okay, go to step  21.
  10. Start engine and allow to idle. Using scan tool, monitor fuel pressure parameter and command HPL open. If fuel pressure increases to more than 1200 psi (8274 kPa) when HPL is commanded open, go to step  15. If fuel pressure does not increase to more than 1200 psi (8274 kPa) when HPL is commanded open, go to next step.
    NOTE: Before performing next step, ensure manual lock-down screw of HPL manual shut-off fitting is opened by rotating Allen screw counter-clockwise until fully seated.
  11. Verify CNG fuel tank is at full pressure by connecting vehicle to a CNG filling station and filling tank to full capacity. Turn ignition on. Compare dispensing station gauge reading to FPS reading on scan tool. If FTP sensor reading is close to dispensing station gauge reading, go to next step. If FTP sensor reading is not close to dispensing station gauge reading, go to FUEL GAUGE INACCURATE/INOPERATIVE (CNG) under SYSTEM TESTS in appropriate ANALOG INSTRUMENT PANELS article.
  12. While monitoring fuel pressure parameter, start engine and allow to idle. Ensure vehicle is operating on alternate fuel. Monitor fuel pressure reading for a few minutes while engine idles. Review test description. See DESCRIPTION . If fuel pressure parameter ever drops to less than 1200 psi (8274 kPa), go to step  19. If fuel pressure parameter does not drop to less than 1200 psi (8274 kPa), go to step  22.
  13. Observe AFO SIGNAL parameter for PCM. Turn ignition off for 30 seconds. Locate AF ECM and disconnect connectors. AF ECM is located behind left front fascia, below headlight. See Figure. Connect a fused jumper wire between AF ECM Blue connector C001 terminal No. 3 (Pink wire) and ground. See Fig 1. Turn ignition on. If fused jumper blows, repair short to voltage in Pink wire. If AFO signal indicates natural gas operation is enabled, go to step  16. If AFO signal does not indicate natural gas operation is enabled, go to next step.
  14. Check for open in Pink wire between AF ECM and PCM connector C1 terminal No. 71. For connector terminal identification, see CONNECTOR IDENTIFICATION in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. Check for poor connections ate PCM. Check that PCM is calibrated with proper bi-fuel information. Repair circuit connections or program PCM as necessary. After repairs, go to step  22. If no problems are found, go to step  17.
  15. Check for fuel system leak or LPL that will not hold pressure when CNG system is closed. Check HPL for proper prime pulse operation. Repair as necessary and go to step  22.
  16. Check for poor connections at AF ECM. Repair as necessary. After repairs, go to step  22. If connections are okay, go to step  18.
  17. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. Program (flash) PCM with correct vehicle software and calibration. See POWERTRAIN CONTROL MODULE REPROGRAMMING under PROGRAMMING in SELF-DIAGNOSTICS - 2.2L CAVALIER CNG article. To perform CKP variation learn procedure, a switch-over to gasoline operation is necessary. Remove CNG fuse (20-amp) located in underhood fuse block to perform switch-over. Perform CKP system variation learn procedure. See CRANKSHAFT POSITION SENSOR SYSTEM VARIATION LEARN PROCEDURE under PROGRAMMING in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. After performing repairs and procedures, go to step  18.
  18. Replace AF ECM. See ALTERNATE FUEL ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. Program (flash) AF ECM with correct vehicle software and calibration. See ALTERNATE FUEL ENGINE CONTROL MODULE under PROGRAMMING in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. After performing repairs and procedures, go to step  18.
  19. Locate and disconnect High Pressure Lock-Off (HPL) solenoid connector. See Figure. Connect test light between HPL solenoid connector terminals "A" (White wire) and "B" (Light Green wire). Using scan tool, command HPL solenoid on and off. If test light illuminates when commanded on, go to next step. If test light does not illuminate when commanded on, go to DTC P1215: GROUND FAULT DETECTION INDICATED under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article.
  20. Replace high pressure lock-off solenoid. See HIGH PRESSURE LOCK-OFF under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. After repairs, go to step  18.
  21. Replace GMS/MCV. See GAS MASS SENSOR/MIXTURE CONTROL VALVE under FUEL SYSTEMS (CNG ENGINES) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. After repairs, go to next step.
  22. Turn ignition off. Remove all test equipment. Reconnect any disconnected components and/or connectors. Using scan tool, clear DTCs from AF ECM and PCM. Turn ignition off for 30 seconds. Start engine and allow to idle. Start engine and operate vehicle noting operation of Fuel Indicator Light (FIL) and Malfunction Indicator Light (MIL). Observe vehicle performance and driveability in gasoline and alternate fuel modes. If FIL or MIL do not illuminate and vehicle performs normally, system is okay at this time. If FIL or MIL illuminate or vehicle does not performs normally, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article.
Fig 1: Identifying Alternate Fuel Engine Control Module Connector C001 Terminals
G00121046Courtesy of GENERAL MOTORS CORP.
Fig 2: Identifying Alternative Fuel Engine Control Module Connector C002 Terminals
G00121047Courtesy of GENERAL MOTORS CORP.
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