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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 Diagnosis >> Diagnostic Procedure
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Diagnostic Procedure

  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. If any DTCs other than P1202 are set, diagnose those DTCs first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article. If no DTCs other than P1202 are set, go to next step.
  2. Connect vehicle to CNG dispensing equipment and verify that CNG tank is at least 1/3 full. This would be a pressure of 1200 psi (8274 kPa). Add CNG to tank as necessary to bring to proper level. Go to next step.
    NOTE: During next step, rhythmic pressure fluctuations of 10-20 psi are normal. A defective HPR is not indicated unless fluctuations become excessive.
  3. Turn ignition off. Remove dust cap from fuel pressure test port located on fuel line near LPR. See Fig 1. Using parts included in CNG Electronic Pressure Gauge Set (J 39970), install Fuel Test Port Adapter (J 39970-100) onto fuel pressure test port. Connect Electronic Fuel Pressure Gauge (J 39970) to adapter. Connect DVOM to gauge. Observe pressure while cranking engine. If a DTC sets that forces a switch-over to gasoline operation (for example: DTC P1202), fuel pressure can only be monitored before DTC sets. If vehicle will not idle in CNG mode, note pressures during cranking. If possible, start engine and allow to idle until engine reaches operating temperature. Record pressure on DVOM at idle. Observe and record pressure while performing the following procedures:
    • Apply parking brake.
    • Press and firmly hold brake pedal.
    • Place vehicle in Drive.
    • Depress throttle to 80 percent (power brake) for maximum of 10 seconds.
    • Release throttle and allow engine to idle in neutral for a minimum of 2 minutes for transmission to cool.

    If recorded pressure values are 115-200 psi (793-1517 kPa), go to step  5. If recorded pressure values are not 115-200 psi (793-1517 kPa), go to next step.

  4. If recorded pressure was more than 220 psi (1517 kPa), go to step  14. If recorded pressure was not more than 220 psi (1517 kPa), go to step  8.
  5. Turn ignition off. Remove LPR primary test port plug. See Fig 1. Connect pressure gauge adapter to LPR primary test port. Connect 0-15 PSI Pressure Gauge (J 28552) to adapter. Observe pressure while cranking engine. If a DTC sets that forces a switch-over to gasoline operation (for example: DTC P1202), fuel pressure can only be monitored before DTC sets. If vehicle will not idle in CNG mode, note pressures during cranking and go to next step. If vehicle will start and idle, run until operating temperature is reached. Record pressure while engine is idling. Observe and record pressure while performing the following procedures:
    • Apply parking brake.
    • Press and firmly hold brake pedal.
    • Place vehicle in Drive.
    • Depress throttle to 80 percent (power brake) for maximum of 10 seconds.
    • Release throttle and allow engine to idle in neutral for a minimum of 2 minutes for transmission to cool.

    If recorded pressure values are 2-64 psi, go to next step. If recorded pressure values are not 2-64 psi, go to step  13.

  6. Turn ignition off. Remove LPR secondary test port plug. A small amount of oil may drain from this port. Connect pressure gauge adapter to secondary test port. Install primary test port plug using teflon sealant, NOT teflon tape. Connect Slack Tube Manometer (J 7333-6) to pressure gauge adapter. Ensure vent port of manometer is closed until engine is started. Start engine and allow to idle until normal operating temperature is reached. Open vent port on manometer. Record manometer reading at idle. Observe and record manometer reading while performing the following procedures:
    • Apply parking brake.
    • Press and firmly hold brake pedal.
    • Place vehicle in Drive.
    • Depress throttle to 80 percent (power brake) for maximum of 10 seconds.
    • Release throttle and allow engine to idle in neutral for a minimum of 2 minutes for transmission to cool.

    If recorded manometer values are 2-5 in. H2 O (5.1-12.7 cm), go to step  19. If recorded manometer values are not 2-5 in. H2 O (5.1-12.7 cm), go to next step.

  7. Check for loose clamps, cracks or deterioration of vapor hoses between LPR and GMS/MCV and between GMS/MCV and Gas Distribution Adapter (GDA). Check GDA for loose or missing components or physical damage. Check that butterfly valve inside GMS/MCV rotates freely and does not drag when rotated. Repair or replace components as necessary. After repairs, go to step  20. If components are okay, go to step  13.
  8. Using scan tool, command LPL on and off. Listen and feel for LPL operation as LPL solenoid is energized and de-energized. If LPL operation is heard or felt when energized, go to next step. If LPL operation is not heard or felt when energized, go to step  15.
  9. Using scan tool, command HPL on and off. Listen for HPL operation as HPL solenoid is energized and de-energized. If HPL operation is heard when energized, go to next step. If HPL operation is not heard when energized, go to step  17.
  10. Turn ignition off. Remove rubber access plug located on fuel tank cover inside trunk. Verify that manual lock-down Allen screw of HPL is fully seated in a counter-clockwise direction (fully open). If manual lock-down Allen screw is not fully open, open as necessary and go to step  20. If manual lock-down Allen screw is fully open, go to next step.
  11. Check fuel line between HPL, HPR and LPR for kinks, bends, damage or obstructions. An obstructed fuel line may be cool or have frost on the surface near obstruction when fuel is flowing through line. Repair or replace as necessary. After repairs, go to step  20. If fuel line is okay, go to next step.
    NOTE: During next step, rhythmic pressure fluctuations of 10-20 psi are normal. A defective HPR is not indicated unless fluctuations become excessive.
  12. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (CNG) under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. Replace HPR filter. See HIGH PRESSURE FUEL REGULATOR under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. Observe pressure while cranking engine. If a DTC sets that forces a switch-over to gasoline operation (for example: DTC P1202), fuel pressure can only be monitored before DTC sets. If vehicle will not idle in CNG mode, note pressures during cranking. If possible, start engine and allow to idle until engine reaches operating temperature. Record pressure at idle. Observe and record pressure while performing the following procedures:
    • Apply parking brake.
    • Press and firmly hold brake pedal.
    • Place vehicle in Drive.
    • Depress throttle to 80 percent (power brake) for maximum of 10 seconds.
    • Release throttle and allow engine to idle in neutral for a minimum of 2 minutes for transmission to cool.

    If recorded pressure values are 115-220 psi (793-1517 kPa), go to step  17. If recorded pressure values are not 115-220 psi (793-1517 kPa), go to step  14.

  13. Replace LPR. See LOW PRESSURE REGULATOR under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. After repairs, go to step  20.
  14. Replace HPR. See HIGH PRESSURE FUEL REGULATOR under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. After repairs, go to step  20.
  15. Disconnect low pressure lock-off solenoid connector. Connect test light between low pressure lock-off solenoid connector terminals "A" (Purple wire) and "B" (Light Blue wire). Using scan tool, command LPL solenoid on and off. If test light turns on when commanded on, go to next step. If test light does not turn on when commanded on, go to DTC P1215: GROUND FAULT DETECTION INDICATED under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article.
  16. Replace low pressure lock-off solenoid. See LOW PRESSURE LOCK-OFF SOLENOID under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. After repairs, go to step  20.
  17. Disconnect high pressure lock-off solenoid connector. Connect test light between high pressure lock-off solenoid connector terminals "A" (White wire) and "B" (Light Green wire). Using scan tool, command HPL solenoid on and off. If test light turns on when commanded on, go to next step. If test light does not turn on when commanded on, go to DTC P1215: GROUND FAULT DETECTION INDICATED under DIAGNOSTIC TESTS in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article.
  18. 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  20.
  19. Observe pressure gauge and turn engine off. If pressure drops to zero within seconds of turning engine off, go to next step. If pressure does not drop to zero within seconds of turning engine off, go to step  16.
  20. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (CNG) under FUEL SYSTEMS (CNG ENGINE) in REMOVAL, OVERHAUL & INSTALLATION - CAVALIER & SUNFIRE article. Turn ignition off. Remove all test equipment. Ensure dust caps and plugs are reinstalled. Use teflon sealant on plugs, NOT teflon tape. Start engine and allow to idle. Using Ultrasonic Leak Detector (J 41416), check for leaks between fuel tank and Low Pressure Regulator (LPR). Check for leaks at LPR and between LPR and Gas Distribution Ring (GDR) using SNOOP® or soapy water. Repair any leaks as necessary. Using scan tool, clear any DTCs from AF ECM and PCM. Turn ignition off and wait 30 seconds. 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 perform normally, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - CNG article.
Fig 1: Identifying CNG Fuel Control Components
G00070992Courtesy of GENERAL MOTORS CORP.
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