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Home >> Pontiac >> 2001 >> Aztek Base, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System & Component Testing >> Fuel System (CNG) >> Fuel System Diagnosis (Alternative Fuel) >> Diagnostic Procedure

Diagnostic Procedure

NOTE: If DTC sets, causing gasoline operation, check pressure only before DTC sets. If vehicle will not idle on CNG, check cranking pressure. A rhythmic pressure fluctuation of 10-20 psi is normal.
  1. Perform OBD system check. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. If any DTCs except P1202 exist, perform appropriate test. See appropriate SELF-DIAGNOSTICS article.
  2. Connect vehicle to filling station. Ensure fuel tank is at least 1/3 full (1200 psi). Go to next step.
  3. Relieve fuel system pressure. See BASIC FUEL SYSTEM CHECKS (CNG) in appropriate BASIC DIAGNOSTIC PROCEDURES article. Remove pressure relief device from High Pressure Regulator (HPR). Install electronic pressure gauge adapter (J-39970-UPD) in HPR. Connect pressure gauge to adapter. Connect DVOM to pressure gauge. Monitor fuel pressure. Start engine. Warm engine to operating temperature. Apply parking brake. Firmly depress brake. Shift vehicle into Drive. Depress throttle to at least 80 percent for 10 seconds. Allow engine to idle for 2 minutes to cool transmission. If fuel pressure does not remain 130-220 psi during testing, go to next step. If fuel pressure remains 130-220 psi during testing, go to step  5.
  4. If pressure is more than 220 psi, go to step  19 If pressure is less than 130 psi, go to step  10.
  5. Turn ignition off. Remove primary test port plug from Low Pressure Regulator (LPR). Install adapter and 0-15 psi pressure gauge from Test Kit (J-42580). Monitor fuel pressure. Start engine. Warm engine to operating temperature. Apply parking brake. Firmly depress brake. Shift vehicle into Drive. Depress throttle to at least 80 percent for 10 seconds. Allow engine to idle for 2 minutes to cool transmission. If fuel pressure does not remain 4-6 psi during testing, go to next step. If fuel pressure remains 4-6 psi during testing, go to step  7.
  6. If pressure is more than 6 psi, go to step  18. If pressure is less than 4 psi, go to step  15.
  7. Turn ignition off. Remove gauge and adapter from primary test port. Install plug, using Teflon® sealant, not tape. Remove secondary test port plug from LPR and install adapter. Some oil may drain from test port. Connect Slack Tube Manometer (J-7333-6) to adapter. Keep vent port of manometer closed until after engine starts. Start engine. Warm engine to operating temperature. Apply parking brake. Firmly depress brake. Shift vehicle into Drive. Depress throttle to at least 80 percent for 10 seconds. Allow engine to idle for 2 minutes to cool transmission. If fuel pressure does not remain 4.5-7.0 in Hg during testing, go to next step. If fuel pressure remains 4.5-7.0 in Hg during testing, go to step  9.
  8. Check for loose clamps, damage, cracks or worn components. Check vapor hose between LPR and GMS/MCV. Check hose between GMS/MCV and Gas Distribution Adapter (GDA). Check for smooth, free operation of butterfly valve in GMS/MCV. If no problem is found, go to step  18. If any problem is found, repair as necessary. After repairs, go to step  22.
  9. Turn ignition off. Remove manometer and adapter from secondary test port. Install plug, using Teflon® sealant, not tape. With DVOM from step  3 and a helper in vehicle, test drive vehicle. Check pressure with vehicle in gear at idle. Check pressure when accelerating with throttle depressed at least 80 percent. If fuel pressure does not remain 130-220 psi during testing, go to step  19. If fuel pressure remains 130-220 psi during testing, go to step  22.
  10. Relieve fuel system pressure. See BASIC FUEL SYSTEM CHECKS (CNG) in appropriate BASIC DIAGNOSTIC PROCEDURES article. Replace filter in High Pressure Regulator (HPR). Monitor fuel pressure. Start engine. Warm engine to operating temperature. Apply parking brake. Firmly depress brake. Shift vehicle into Drive. Depress throttle to at least 80 percent for 10 seconds. Allow engine to idle for 2 minutes to cool transmission. If fuel pressure does not remain 130-220 psi during testing, go to next step. If fuel pressure remains 130-220 psi during testing, go to step  22.
  11. Turn ignition off. Remove fuel tank cover. Disconnect 2-pin HPL connector C313. Connect test light between harness connector terminals. Crank engine. If test light illuminates, go to next step. If test light does not illuminate, check relay. See LOCK-OFF RELAY .
  12. Ensure manual lock-down screw on HPL manual shut-off fitting is open by turning Allen screw fully counter-clockwise. If shut off valve was open, go to next step. If shut off valve was not fully open, open valve and go to step  22.
  13. Turn ignition off. Connect HPL connector. AF ECU should energize HPL for one second when ignition is turned on. Check HPL operation while turning ignition on. If HPL is okay, go to next step. If HPL does not operate, go to step  20.
    14. NOTE: An obstruction in the fuel line will cause cooling or frosting after the restriction.
  14. Check for kinks, bends, damage and obstruction in fuel line between HPL and HPR. If no problem is found, go to step  19. If any problem is found, repair as necessary. After repairs, go to step  22.
  15. Turn ignition off. Disconnect 2-pin LPL connector C014. Connect test light between harness connector terminals. Crank engine. If test light illuminates, go to next step. If test light does not illuminate, check relay. See LOCK-OFF RELAY .
  16. Connect switch box from CNG Tank Venting Kit (J-42435) to LPL and vehicle battery. Using switch box, turn LPL on and off. If LPL operates, go to next step. If LPL does not operate, go to step  21.
  17. Check for kinks, bends, damage and obstruction in fuel line between LPL and HPR. If no problem is found, go to next step. If any problem is found, repair as necessary. After repairs, go to step  22.
  18. Replace low pressure regulator. After repairs, go to step  22.
  19. If sent from step  9, check for kinks, bends, damage and obstruction in fuel line between HPL and HPR. Repair as necessary. If no problem is found or sent from any other step, replace high pressure regulator. After repairs, go to step  22.
  20. Replace high pressure lock-off solenoid. After repairs, go to step  22.
  21. Replace low pressure lock-off solenoid. After repairs, go to next step.
  22. Relieve fuel system pressure. See BASIC FUEL SYSTEM CHECKS (CNG) in appropriate BASIC DIAGNOSTIC PROCEDURES article. Turn ignition off. Remove all test equipment. Ensure PRD and test port plugs are installed using Teflon® sealant, not tape. Star engine. Check for system leaks. Repair any leaks as necessary. Using scan tool, clear DTCs from AF ECU and VCM. Turn ignition off for 30 seconds. Start engine. Test drive vehicle. If driveability is okay and MIL does not illuminate, test is complete. If driveability problems exist and/or MIL illuminates, check system. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
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