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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 220 (ENGINE CONTROLS - BASIC DIAGNOSTIC PROCEDURES - 6.0L W/BiFuel & CNG ENGINES) >> Fuel Systems >> Fuel System Diagnosis >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 GMC Sierra, 2003 GMC Savana, 2003 Chevrolet Silverado, and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check. See DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG or DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article. After performing diagnostic system check, go to next step.
  2. Connect 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 with a scan tool. Attempt to start and idle the engine. Does the FTP sensor display a steady pressure at or greater than 1200 psi (8274 kPa)? If yes, go to step next. If no, go to step  6.
  3. Attempt to start and idle engine. Operate engine at idle until normal operating temperature is obtained. If engine will not start on CNG, use the pressure values observed during cranking. Observe and record the FRP sensor display with a scan tool. Compare the recorded scan tool display pressure values to the value range of 130-200 psi (896-1379 kPa)? Is FRP sensor value within the specified range? If yes, go to next step. If no, go to step  8.
  4. Turn ignition off. Turn ignition on, with engine off. Command the LPL solenoid ON and OFF with a scan tool in order to charge the fuel rail with pressure. Monitor the fuel rail pressure for 5 minutes after commanding the LPL solenoid closed. Does the fuel rail pressure drop more than 3 psi (21 kPa)? If yes, go to step  10. If no, go to next step.
  5. If engine will not start on CNG, use the pressure values observed during cranking. Observe FRP sensor display with the scan tool while cranking the engine. Operate engine at idle 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. With engine idling, observe and record the FRP sensor display on scan tool. Observe and record the pressure indicated by the FRP sensor display during engine load by performing the following steps: Apply parking brake. Firmly apply and hold brake pedal. Place vehicle in forward gear. Observe pressure while depressing throttle to 75 percent or greater for a maximum of 10 seconds. Allow engine to idle for a minimum of 2 minutes to cool transmission. Compare the recorded pressure values to the value range of 130-200 psi (896-1379 kPa). Are all of the fuel pressure values within the specified range? If yes, problem is intermittent. If no, go to step  12.
  6. Turn ignition on, with engine off. Command the HPL on and off with scan tool. Listen for operation of the HPLs. HPL is located on CNG tank. Can operation of the HPLs be heard? If yes, go to next step. If no, diagnose fuel lock-off relay. See ALTERNATIVE FUEL LOCK-OFF RELAY under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  7. Ensure manual tank valves are fully open by turning the screws fully counterclockwise. See Figure. Did you have to open the manual tank valves? If yes, go to step  2. If no, go to step  14.
  8. Is the fuel pressure greater than 200 psi (1379 kPa)? If yes, go to step  15. If no, go to next step.
  9. Turn ignition on, with engine off. Command the LPL on and off with scan tool. Listen for operation of the LPL. LPL is located on left engine valve cover. Can operation of the LPL be heard? If yes, go to step  12. If no, diagnose AF fuel lock-off relay. Relay is attached to underhood fuse/relay block. See ALTERNATIVE FUEL LOCK-OFF RELAY under MODULES, MOTORS, RELAYS & SOLENOIDS in SYSTEM & COMPONENT TESTING - 6.0L CHEVY EXPRESS, SAVANA, SIERRA & SILVERADO - BI-FUEL & CNG article.
  10. Inspect for a fuel system leak between the LPL solenoid and CNG fuel injectors. Replace any leaking components as necessary. Did you find and correct the condition? If yes, go to step  16. If no, go to next step.
  11. Raise CNG fuel rails, lines and injectors as an assembly. Command the LPL on with scan tool. Locate and replace the leaking CNG fuel injector. After repairs, go to step  16.
  12. Inspect the fuel line between the HPL and HPR and the fuel line between the HPR and LPL for kinks, bends, 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  16. If no, go to next step.
  13. Release CNG fuel system pressure. Replace CNG in-line filter. Replace the HPR filter. Observe the FRP sensor display with a scan tool while cranking engine. Operate 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 FRP sensor display at engine idle. Observe and record the FRP sensor display during engine load by performing the following steps: Apply parking brake. Firmly depress and hold brake pedal. Place 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 2 minutes to cool the transmission. Compare the recorded pressure values to the value range of 130-200 psi (896-1379 kPa). Are all fuel pressure values within the specified range? If yes, go to step  16. If no, go to step  15.
  14. Replace FTP sensor. FTP sensor is threaded into HPL, and threaded into CNG tank end. After replacing sensor, go to step  16.
  15. Replace the HPR. After replacing HPR, go to next step.
  16. Release CNG fuel system pressure. Turn ignition off. Remove all remaining test equipment. Start and operate engine at idle. Inspect for CNG fuel system leaks using an Ultrasonic Leak Detector (J-41416) or soapy water. Repair any leaks as necessary. Clear DTC information from PCM with a scan tool. Turn ignition off and wait 30 seconds. Start engine and operate vehicle. Observe MIL. Observe vehicle performance and driveability. Does the vehicle operate on CNG with normal driveability and no MIL illumination? If yes, system is okay. If no, see DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L CHEVY EXPRESS & SAVANA - BI-FUEL & CNG article or DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 6.0L SILVERADO & SIERRA - BI-FUEL & CNG article.
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