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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 165 (ENGINE CONTROLS - BASIC DIAGNOSTIC PROCEDURES - 2.2L)FUEL SYSTEMSFUEL SYSTEM DIAGNOSIS (CNG)DIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls (Alternative Fuels). If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS (ALTERNATIVE FUELS) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - BI-FUEL - CNG/GASOLINE article.
  2. Connect the vehicle to a CNG dispensing station and verify that the CNG fuel tank is at least 1/3 full. Fill CNG fuel tank if necessary. Observe the Fuel Tank Pressure (FTP) sensor data display using a scan tool. Attempt to start and idle the engine. Does the FTP sensor display a steady pressure at or above 1200 psi (8274 kPa)? If yes, go to next step. If no, go to step  6.
    3. NOTE: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the step question.
  3. Attempt to start and idle the engine. Idle the engine until normal operating temperature is obtained. Observe and record the FRP sensor display using a scan tool. Compare the recorded scan tool display pressure values to the value range specified. Is the FRP sensor value 40-50 psi (276-345 kPa)? If yes, go to next step. If no, go to step  8.
  4. Turn ignition switch to OFF position. Turn ignition switch to RUN position, engine OFF. Monitor the fuel rail pressure for 5 minutes. Does the fuel rail pressure drop more than 3 psi (21 kPa)? If yes, go to step  11. If no, go to next step.
    5. NOTE: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the step question.
  5. Turn ignition switch to OFF position. Remove the dust cap from the LPR pressure test port. Depress the valve core stem to relieve CNG fuel pressure from the LPR. Using the components available within the CNG Electronic Pressure Gauge Kit (J 39970), or equivalent, install the transducer to the LPR test port. Connect the CNG electronic pressure gauge to the Adapter (J 39970-100), or equivalent. Connect the DMM to the CNG electronic pressure gauge. Zero the DMM using the procedure included with the CNG electronic pressure gauge. Observe the fuel pressure using the DMM while cranking the engine. Idle the engine until normal operating temperature is obtained. Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the pressure indicated by the DMM at engine idle. Observe and record the pressure indicated by the DMM during engine load by performing the following steps:
    • Apply the parking brake.
    • Firmly apply and hold the brake pedal.
    • Place the vehicle in forward gear.
    • Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    • Allow the engine to idle for a minimum of two minutes to cool the transmission.

    Compare the recorded pressure values to the value range specified. Are all of the fuel pressure values 110-210 psi (758-1448 kPa)? If yes, go to INTERMITTENTS in TROUBLE SHOOTING - 2.2L CAVALIER - BI-FUEL - CNG/GASOLINE article.

  6. Turn ignition switch to RUN position, engine OFF. Command the HPL ON and OFF using a scan tool. Listen for operation of the HPL. Can operation of the HPL be heard? If yes, go to next step. If no, go to step  19.
  7. Remove the vent boot plug. Ensure the manual tank valve is fully open by turning the screw fully counter clockwise. Did you have to open the manual tank valve? If yes, go to step  2. If no, go to step  15.
    8. NOTE: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the step question.
  8. Turn ignition switch to OFF position. Remove the dust cap from the LPR pressure test port. Depress the valve core stem to relieve CNG fuel pressure from the LPR. Using the components available within the CNG Electronic Pressure Gauge Kit (J 39970), install the transducer to the pressure test port. Connect the CNG electronic pressure gauge to the Adapter (J 39970-100), or equivalent. Connect the DMM to the CNG Electronic Pressure Gauge. Zero the DMM using the procedure included with the CNG electronic pressure gauge. Observe the fuel pressure using the DMM, while cranking the engine. Idle the engine until normal operating temperature is obtained. Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the pressure indicated by the DMM at engine idle. Compare the recorded pressure value to the value range specified. Is the pressure value 110-210 psi (758-1448 kPa)? If yes, go to step  10. If no, go to next step.
  9. Is the fuel pressure greater than 210 psi (1448 kPa)? If yes, go to step  17. If no, go to step  13.
  10. Turn ignition switch to RUN position, engine OFF. Command the LPL solenoid ON and OFF using a scan tool. Listen for operation of the LPL. Can operation of the LPL be heard? If yes, go to step  16. If no, go to step  18.
  11. Inspect for a fuel system leak between the LPL solenoid and the CNG fuel injectors. Replace any leaking components as necessary. Did you find and correct the condition? If yes, go to step  20. If no, go to next step.
  12. Raise the CNG fuel rail, lines and injectors as an assembly. Command the LPL ON using a scan tool. Locate and replace the leaking CNG fuel injector. After repair, go to step  20.
  13. Inspect the fuel line between the HPL and the HPR and the fuel line between the HPR and the LPL for kinks, bend, damage or obstruction. An obstructed fuel line may be cool or frosted near the obstruction when fuel flows through the line. Repair or replace components as necessary. Did you find and correct the condition? If yes, go to step  20. If no, go to next step.
    14. NOTE: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the step question.
  14. Relieve the CNG fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE . Replace the CNG In-line filter. Replace the HPR filter. Observe the fuel pressure using the DMM while cranking the engine. Idle the engine until normal operating temperature is obtained. Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Observe and record the pressure indicated by the DMM at engine idle. Observe and record the pressure indicated by the DMM during engine load by performing the following steps:
    • Apply the parking brake.
    • Firmly depress and hold the brake pedal.
    • Place the vehicle in forward gear.
    • Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    • Allow the engine to idle for a minimum of two minutes to cool the transmission.

    Compare the recorded pressure values to 110-210 psi (758-1448 kPa). Are all fuel pressure values 110-210 psi (758-1448 kPa)? If yes, go to step  20. If no, go to step  17.

  15. Replace the FTP sensor. After repair, go to step  20.
  16. Replace the LPR. After repair, go to step  20.
  17. Replace the HPR. After repair, go to step  20.
  18. Replace the LPL solenoid. After repairs, go to step  20.
  19. Replace the HPL solenoid. After repairs, go to next step.
  20. Relieve the CNG fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE . Turn ignition switch to OFF position. Remove all remaining test equipment. Install the IPR test port dust cap. Start and idle the engine. Inspect for CNG fuel system leaks between the fuel tank and the IPR using the Ultrasonic Leak Detector (J 41416), or equivalent. Inspect for CNG fuel system leaks at the fuel rails and injectors using Snoop® or soapy water. Repair any leaks as necessary. Clear DTC information using a scan tool. Turn ignition switch to OFF position and wait 30 seconds. Start the engine and operate the vehicle. Observe the MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? If yes, system is okay at this time. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS (ALTERNATIVE FUELS) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - BI-FUEL - CNG/GASOLINE article.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.