GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 GMC C Sierra - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel System Diagnosis

System Description

The powertrain control module (PCM) controls the fuel system while the vehicle is operating on CNG. The PCM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in a cylindrical fuel tank. A full tank can contain fuel at 24,820 kPa (3600 psi). A high pressure lock-off (HPL) solenoid in the tank and a low pressure lock-off (LPL) solenoid in the fuel line prevent fuel flow. The PCM commands ONLY the HPL open for 1 second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tank. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

This fuel delivery system utilizes two different pressure stages in order to lower the fuel pressure from tank pressure down to an injector delivery pressure.

High Pressure Regulator Stage

When the PCM commands the HPL open fuel begins to flow through the high pressure regulator (HPR). The HPR reduces fuel pressure to between 758-1448 kPa (110-210 psi). The outlet of the HPR is the intermediate pressure stage.

Intermediate Pressure Regulator Stage

With a pressure between 758-1448 kPa (110-210 psi) the fuel exiting the HPR flows through the intermediate fuel line to the low pressure lock-off (LPL) solenoid. The LPL is controlled by the PCM. Fuel flows out of the LPL and into the intermediate pressure regulator (IPR). The IPR reduces fuel pressure to between 246-317 kPa (43-46 psi). Fuel flows out of the IPR and into the fuel rails.

Test Description

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

  1. This step verifies that the HPL solenoid and the FTP sensor are functioning. A fuel tank pressure value that decreases while the engine is operating indicates the HPL is not opening or the manual tank valve is closed.

    2.

  2. This step verifies the FRP sensor is displaying the correct fuel pressure.

    3.

  3. This step tests for a leak in the fuel system between the low pressure lock-off (LPL), and the fuel injectors. The fuel pressure should remain constant during the test.

    4.

  4. 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.

  5. This step tests the operation of the HPL.

    6.

  6. 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.

    7.

  7. This step determines if the FRP sensor or actual fuel rail pressure is the cause of the condition.

    8.

  8. 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.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

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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 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 fuel rail pressure for 5 minutes.

Does the fuel rail pressure 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 IPR pressure test port.
  3. Depress the valve core stem in order to relieve CNG fuel pressure from the IPR.
  4. Using the components available within the J 39970 CNG Electronic Pressure Gage kit, install the transducer to the IPR test port.
  5. Connect the J 39970 to the J 39970-100 .
  6. Connect the J 39200 DMM to the J 39970 .
  7. Zero the J 39200 DMM using the procedure included with the J 39970 .

    8. Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.

  8. Observe the fuel pressure with the DMM while cranking the engine.
  9. Idle the engine until normal operating temperature is obtained.

    10. Important: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.

  10. Observe and record the pressure indicated by the DMM at engine idle.
  11. Observe and record the pressure indicated by the DMM during engine load by performing the following steps:
    12. 11.1. Apply the parking brake.
    11.2. Firmly apply and hold the brake pedal.
    11.3. Place the vehicle in forward gear.
    11.4. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    11.5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  12. 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 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 Step 20

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 OFF the ignition.
  2. Remove the dust cap from the IPR pressure test port.
  3. Depress the valve core stem in order to relieve CNG fuel pressure from the IPR.
  4. Using the components available within the J 39970 CNG Electronic Pressure Gage kit, install the transducer to the pressure test port.
  5. Connect the J 39970 to the J 39970-100 .
  6. Connect the J 39200 DMM to the J 39970 .
  7. Zero the J 39200 DMM using the procedure included with the J 39970 .

    8. Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question.

  8. Observe the fuel pressure with the DMM while cranking the engine.
  9. Idle the engine until normal operating temperature is obtained.

    10. Important: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.

  10. Observe and record the pressure indicated by the DMM at engine idle.
  11. 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 10

Go to Step 9

9

Is the fuel pressure greater than the specified value?

1 448 kPa (210 psi)

Go to Step 18

Go to Step 14

10
  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 11

Go to Step 19

11

  1. Replace the FRP sensor. Refer to Fuel Injection Fuel Rail Fuel Pressure Sensor Replacement .
  2. Attempt to start and idle the engine.
  3. Idle the engine until normal operating temperature is obtained.

    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 and record the FRP sensor display with a scan tool.
  5. Compare the recorded scan tool display pressure values to the value range specified.

Is the FRP sensor pressure within the specified value range?

276-345 kPa (40-50 psi)

Go to Step 21

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 21

Go to Step 13

13
  1. Raise the CNG fuel rails, lines and injectors as an assembly. Refer to Fuel Injection Fuel Rail Assembly Replacement .
  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 21

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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:
    2. • 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 21

Go to Step 15

15

  1. Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief .
  2. Replace the CNG In-line filter. Refer to Fuel Filter Replacement .
  3. Replace the HPR filter. Refer to High Pressure Regulator 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:
    8. 7.1. Apply the parking brake.
    7.2. Firmly depress and hold the brake pedal.
    7.3. Place the vehicle in forward gear.
    7.4. Observe the pressure while depressing the throttle to 75 percent or greater for a maximum of 10 seconds.
    7.5. 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 21

Go to Step 18

16

Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 21

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17

Replace the IPR. Refer to Intermediate Pressure Regulator Replacement .

Did you complete the replacement?

--

Go to Step 21

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18

Replace the HPR. Refer to Compressed Natural Gas High Pressure Regulator Replacement .

Did you complete the replacement?

--

Go to Step 21

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19

Replace the LPL solenoid. Refer to Low Pressure Lock-Off Solenoid Replacement .

Did you complete the replacement?

--

Go to Step 21

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20

Replace the HPL solenoid. Refer to High Pressure Lock-Off Solenoid Replacement .

Did you complete the replacement?

--

Go to Step 21

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21
  1. Relieve the CNG fuel system pressure. Refer to Fuel Pressure Relief .
  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?

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System OK

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)