GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC K Sierra - 4WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel System Diagnosis

System Description

The alternative fuels engine control module (ECM) controls the fuel system while the vehicle is operating on CNG. The alternative fuels ECM 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 alternative fuels ECM 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 alternative fuels ECM 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 alternative fuels ECM 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 alternative fuels ECM. 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 number below refers to the step number on the diagnostic table.

  1. This step verifies that there is adequate fuel in the CNG fuel tank.

    2.

  2. This step verifies the fuel rail pressure (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 high pressure regulator (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 high pressure lock-off (HPL). Cycle the ignition quickly from lock to run as you listen for HPL operation.

    6.

  6. This step tests for an electrical condition that is causing an inoperative HPL. If the test lamp does not illuminate, go to DTC P1215 for diagnosis of the electrical circuits.

    7.

  7. This step tests the high pressure regulator (HPR) output pressure. If the fuel pressure is within the specified values, then the HPR, fuel lines, and filters are OK.

    8.

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

    9.

  9. This step tests for an electrical condition that is causing an inoperative LPL. If the test lamp does not illuminate, go to DTC P1215 for diagnosis of the electrical circuits.

    10.

  10. 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

Perform the Alternative Fuels Diagnostic System Check-Engine Controls?

Are alternative fuels ECM DTCs P0201-P0208, P0602, P0605 or P1215 set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 2

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.

Does the FTP sensor display a pressure at or above the specified value?

8274 kPa (1200 psi )

Go to Step 3

Go to Step 6

3
  1. Observe the fuel rail pressure (FRP) sensor display with a scan tool.
  2. Observe the FRP sensor display while cranking the engine.

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

  3. Idle the engine until normal operating temperature is obtained.
  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 value within the specified range?

276-345 kPa (40-50 psi)

Go to Step 4

Go to Step 9

4
  1. Turn ON the ignition with the engine OFF.
  2. Observe and record the fuel rail pressure immediately after cranking the engine for a few seconds. Turn the engine OFF if it starts.
  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 14

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Remove the dust cap from the pressure test port located on the intermediate pressure regulator (IPR).
  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 adaptor with the J 39970-100 Fuel Test Port Adaptor onto 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. Observe the fuel pressure with the DMM while cranking the engine.

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

  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?

758-1448 kPa (110-210 psi )

Go to Intermittent Conditions

Go to Step 17

6

Listen for high pressure lock-off (HPL) solenoid operation as you quickly turn the ignition between LOCK and RUN.

Can operation of the HPL be heard?

--

Go to Step 7

Go to Step 8

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 18

8
  1. Remove the fuel tank cover. Refer to Fuel Tank Cover Replacement .
  2. Disconnect the harness connector of the HPL solenoid.
  3. Connect the J 35616-200 test lamp between the body harness HPL connector terminals.
  4. Observe the test lamp as you turn ON the ignition, with the engine OFF.

Does the test lamp illuminate for about 2 seconds?

--

Go to Step 22

Go to DTC P1215

9
  1. Turn OFF the ignition.
  2. Remove the dust cap from the pressure test port located on the intermediate pressure regulator (IPR).
  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 adaptor with the J 39970-100 Fuel Test Port Adaptor onto 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. Observe the fuel pressure with the DMM while cranking the engine.

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

  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?

758-1448 kPa (110-210 psi)

Go to Step 11

Go to Step 10

10

Is the fuel pressure greater than the specified value?

1448 kPa (210 psi)

Go to Step 20

Go to Step 16

11
  1. Turn OFF the ignition.
  2. Feel the low pressure lock-off (LPL) solenoid as an assistant cranks the engine.

Can you feel the LPL operate as the engine is cranking?

--

Go to Step 12

Go to Step 13

12
  1. Turn OFF the ignition.
  2. Replace the FRP sensor. Refer to Fuel Injection Fuel Rail Fuel Pressure 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 pressure within the specified value range?

276-345 kPa (40-50 psi)

Go to Step 23

Go to Step 19

13
  1. Turn OFF the ignition.
  2. Disconnect the harness connector of the low pressure lock-off (LPL) solenoid.
  3. Connect the J 35616-200 test lamp between the engine harness LPL connector terminals.
  4. Observe the test lamp as you turn crank the engine.

Does the test lamp illuminate during cranking?

--

Go to Step 21

Go to DTC P1215

14
  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 23

Go to Step 15

15
  1. Raise the CNG fuel rails, lines and injectors as an assembly. Refer to Fuel Injection Fuel Rail Assembly Replacement .
  2. Remove the IGN E fuse.
  3. Crank the engine for the specified time in order to charge the CNG fuel rail with pressure.
  4. Locate and replace the leaking CNG fuel injector. Refer to Fuel Injector Replacement .

Did you complete the replacement?

5 seconds

Go to Step 23

--

16
  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 23

Go to Step 17

17
  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and returning to this step.
  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. Observe the fuel pressure with the DMM while cranking the engine.

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

  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?

758-1448 kPa (110-210 psi )

Go to Step 23

Go to Step 20

18

Replace the fuel tank pressure (FTP) sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Did you complete the replacement?

--

Go to Step 23

--

19

Replace the intermediate pressure regulator (IPR). Refer to Intermediate Pressure Regulator Replacement .

Did you complete the replacement?

--

Go to Step 23

--

20

Replace the high pressure regulator (HPR). Refer to Compressed Natural Gas High Pressure Regulator Replacement .

Did you complete the replacement?

--

Go to Step 23

--

21

Replace the low pressure lock-off (LPL) solenoid. Refer to Low Pressure Lock-Off Solenoid Replacement .

Did you complete the replacement?

--

Go to Step 23

--

22

Replace the high pressure lock-off (HPL) solenoid. Refer to High Pressure Lock-Off Solenoid Replacement .

Did you complete the replacement?

--

Go to Step 23

--

23
  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and returning to this step.
  2. Turn OFF the ignition.
  3. Remove all remaining test equipment.
  4. Ensure the dust cap is re-installed onto the under-hood test port.
  5. Start and idle the engine.
  6. Inspect for CNG fuel system leaks between the fuel tank and the IPR using the J 41416 Ultrasonic Leak Detector.
  7. Inspect for CNG fuel system leaks at the fuel rails and injectors using Snoop® or soapy water.
  8. Repair any leaks as necessary.
  9. Using a scan tool clear DTC information from the alternative fuels ECM and from the PCM.
  10. Turn the ignition OFF and wait 30 seconds.
  11. Start the engine and operate the vehicle.
  12. Observe the MIL.
  13. 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)