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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1172 (Engine Controls - 6.0L - Alternative Fuels (Troubleshooting)) >> Troubleshooting >> Fuel System Diagnosis >> Test Description

Test Description

WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

  1. 2:  This step verifies that the HPL solenoid and the FTP sensor are functioning. If the HPL solenoid is not functioning or if the manual tank valve is closed the dispensing station will charge only the fuel lines with pressure. Cranking or starting the engine will cause this pressure to decrease rapidly.
  2. 3:  This step verifies the fuel rail pressure (FRP) sensor is displaying the correct fuel pressure.
  3. 4:  This step tests for a leak in the fuel system between the LPL, and the fuel injectors. The fuel pressure should remain constant during the test.
  4. 5:  This step tests the HPR output pressure. If the fuel pressure is within the specified values, then the HPR, fuel lines, and filters are OK.
  5. 6:  This step tests the operation of the HPL.
  6. 8:  This step tests the operation of the LPL.
  7. 15:  This step verifies that a fuel filter is not the cause of the condition. If the fuel pressure is within the specified range after the fuel filter replacement, then a plugged filter was the cause of the condition.
Fuel System Diagnosis

Step Action Value(s) Yes No
Schematic Reference: Engine Controls Schematics
1 Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Alternative Fuels (Engine Controls)
2 
  1. Connect the vehicle to a CNG dispensing station and verify that the CNG fuel tank is at least 1/3 full.
  2. Fill CNG fuel tank if necessary.
  3. Observe the fuel tank pressure (FTP) sensor data display with a scan tool.
  4. Attempt to start and idle the engine.
Does the FTP sensor display a steady pressure at or above the specified value?		 8,274 kPa (1,200 psi) Go to Step 3  Go to Step 6 
3 
  1. Attempt to start and idle the engine.
  2. Idle the engine until normal operating temperature is obtained.
    3. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.
  3. Observe and record the fuel rail pressure (FRP) sensor display with a scan tool.
  4. Compare the recorded scan tool display pressure values to the value range specified.
Is the FRP sensor value within the specified range?		 276-345 kPa (40-50 psi) Go to Step 4  Go to Step 8 
4 
  1. Turn OFF the ignition.
  2. Turn ON the ignition with the engine OFF.
  3. Monitor the FRP for 5 minutes.
Does the FRP drop more than the specified value?		 21 kPa (3 psi) Go to Step 12  Go to Step 5 
5 
  1. Turn OFF the ignition.
  2. Remove the dust cap from the intermediate pressure regulator (IPR) pressure test port.
  3. Using the components available within the J 39970  CNG Electronic Pressure Gage kit, install the transducer to the IPR test port.
  4. Connect the J 39970  to the J 39970-100  Fuel Test Port Adapter.
  5. Connect the J 39200  DMM to the J 39970  .
  6. Zero the J 39200  DMM using the procedure included with the J 39970  .
    7. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.
  7. Observe the fuel pressure with the DMM while cranking the engine.
  8. Idle the engine until normal operating temperature is obtained.
    9. IMPORTANT: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective high pressure regulator (HPR) is not indicated unless the fluctuations become excessive.
  9. Observe and record the pressure indicated by the DMM at engine idle.
  10. Observe and record the pressure indicated by the DMM during engine load by performing the following steps:
    1. Apply the parking brake.
    2. Firmly apply and hold the brake pedal.
    3. Place the vehicle in forward gear.
    4. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  11. Compare the recorded pressure values to the value range specified.
Are all of the fuel pressure values within the specified range?		 758-1,448 kPa (110-210 psi) Go to Intermittent Conditions  Go to Step 14 
6 
  1. Turn ON the ignition with the engine OFF.
  2. Command the high pressure lock-off (HPL) ON and OFF with a scan tool.
  3. Listen for operation of the HPL.
Can operation of the HPL be heard?		 - Go to Step 7  Go to DTC P0005
7
  1. Remove the fuel tank cover. Refer to Fuel Tank Cover Replacement .
  2. Ensure the manual tank valve is fully open by turning the screw fully counter clockwise.
Did you have to open the manual tank valve?		 - Go to Step 2  Go to Step 16 
8 
  1. Turn the ignition ON with the engine OFF.
  2. Command the low pressure lock-off (LPL) ON and OFF with a scan tool.
  3. Listen for operation of the LPL.
Can operation of the LPL be heard?		 - Go to Step 9  Go to DTC P2665
9
  1. Turn OFF the ignition.
  2. Remove the dust cap from the IPR pressure test port.
  3. Using the components available within the J 39970  , install the transducer to the pressure test port.
  4. Connect the J 39970  to the J 39970-100  .
  5. Connect the J 39200  DMM to the J 39970  .
  6. Zero the J 39200  DMM using the procedure included with the J 39970  .
    7. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.
  7. Observe the fuel pressure with the DMM while cranking the engine.
  8. Idle the engine until normal operating temperature is obtained.
    9. IMPORTANT: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.
  9. Observe and record the pressure indicated by the DMM at engine idle.
  10. Compare the recorded pressure value to the value range specified.
Is the pressure value within the specified range?		 758-1,448 kPa (110-210 psi) Go to Step 11  Go to Step 10 
10 Is the fuel pressure greater than the specified value? 1,448 kPa (210 psi) Go to Step 18  Go to Step 14 
11
  1. Turn the ignition ON with the engine OFF.
  2. Replace the FRP sensor. Refer to Fuel Rail Pressure (FRP) Sensor Replacement .
  3. Attempt to start and idle the engine.
  4. Idle the engine until normal operating temperature is obtained.
    5. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.
  5. Observe and record the FRP sensor display with a scan tool.
  6. Compare the recorded scan tool display pressure values to the value range specified.
Is the FRP sensor voltage within the specified value range?		 276-345 kPa (40-50 psi) Go to Step 19  Go to Step 17 
12
  1. Inspect for a fuel system leak between the LPL solenoid and the CNG fuel injectors.
  2. Replace any leaking components as necessary.
Did you find and correct the condition?		 - Go to Step 19  Go to Step 13 
13
  1. Raise the CNG fuel rails, lines and injectors as an assembly. Refer toFuel Rail Assembly Replacement (KL6) or Fuel Rail Assembly Replacement (KL8) .
  2. Command the LPL ON with a scan tool.
  3. Locate and replace the leaking CNG fuel injector. Refer to Fuel Injector Replacement .
Did you complete the replacement?		 - Go to Step 19  -
14
  1. Inspect the fuel line between the HPL and the HPR and the fuel line between the HPR and the LPL for the following:
    • Kinks
    • Bends
    • Damage
    • Obstruction-An obstructed fuel line may be cool or frosted near the obstruction when fuel flows through the line.
  2. Repair or replace components as necessary.
Did you find and correct the condition?		 - Go to Step 19  Go to Step 15 
15 
  1. Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief Procedure .
  2. Replace the CNG in-line filter. Refer to Fuel Filter Replacement .
  3. Replace the HPR filter. Refer to High Pressure Regulator (HPR) Filter Replacement .
    4. IMPORTANT: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.
  4. Observe the fuel pressure with the DMM while cranking the engine.
  5. Idle the engine until normal operating temperature is obtained.
    6. IMPORTANT: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.
  6. Observe and record the pressure indicated by the DMM at engine idle.
  7. Observe and record the pressure indicated by the DMM during engine load by performing the following steps:
    1. Apply the parking brake.
    2. Firmly depress and hold the brake pedal.
    3. Place the vehicle in forward gear.
    4. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  8. Compare the recorded pressure values to the value range specified.
Are all fuel pressure values within the specified range?		 758-1,448 kPa (110-210 psi) Go to Step 19  Go to Step 18 
16 Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement .
Did you complete the replacement?		 - Go to Step 19  -
17 Replace the IPR. Refer to Intermediate Pressure Regulator (IPR) Replacement .
Did you complete the replacement?		 - Go to Step 19  -
18 Replace the HPR. Refer to High Pressure Regulator (HPR) Replacement .
Did you complete the replacement?		 - Go to Step 19  -
19
  1. Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief Procedure .
  2. Turn OFF the ignition.
  3. Remove all remaining test equipment.
  4. Install the IPR test port dust cap.
  5. Start and idle the engine.
  6. Inspect for CNG fuel system leaks with the J 45878  Combustible Gas Detector.
  7. Repair any leaks as necessary.
  8. Clear DTC information with a scan tool from the PCM.
  9. Turn the ignition OFF and wait 30 seconds.
  10. Start the engine and operate the vehicle.
  11. Observe the MIL.
  12. Observe vehicle performance and driveability.
Does the vehicle operate on CNG with normal driveability and no MIL illumination?		 - System OK Go to Diagnostic System Check - Alternative Fuels (Engine Controls)
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