GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Pickup - 4WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Diagnosis

Fuel Pressure Gauge Connection
Object Number: 399675 Size: MF

System Description

The alternative fuel engine control unit (AF ECU) controls the fuel system while the vehicle is operating on alternative fuel. The AF ECU 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 3600 psi (24,820 kPa). 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 AF ECU 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 pressure sensor (FPS) to monitor the amount of fuel pressure in the tank. The AF ECU 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 three different pressure stages in order to lower the fuel pressure from tank pressure down to a delivery pressure which is slightly above atmospheric pressure.

High Pressure (tank) Stage

When the AF ECU commands the HPL open fuel begins to flow (at up to 3600 psi) through the high pressure regulator (HPR). The HPR reduces fuel pressure to between 130 and 220 psi. The outlet of the HPR is the intermediate pressure stage.

Intermediate Pressure Stage

With a pressure between 130 and 220 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 AF ECU. Fuel flows out of the LPL through another portion of intermediate fuel line and into the low pressure regulator (LPR). The inlet of the LPR is the Low Pressure Stage.

Low Pressure Stage

The low pressure regulator is a two stage regulator. The fuel enters the LPR at the intermediate stage pressure of 130 to 220 psi. The Primary stage of LPR operation reduces the fuel pressure down to between 4 and 6 psi. The fuel pressure is then again reduced in the Secondary stage of the LPR down to between 4.5 and 7 inches of water column. The fuel exits the LPR and flows through the low pressure hose and into the gas mass sensor/mixture control valve (GMS / MCV).

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This step checks the fuel pressure output of the high pressure regulator (HPR).

    2.

  2. This step checks the fuel pressure output of the Primary Stage of the low pressure regulator (LPR).

    3.

  3. CNG fuel may contain small amounts of fill station compressor oil. As the CNG fuel pressure drops in the LPR secondary chamber this oil separates from the CNG fuel and collects in the secondary chamber. Allow the oil to drain as this condition is normal and no repair is required.

    This step checks the fuel pressure output of the Secondary Stage of the low pressure regulator.

    4.

  4. This step performs a road-load test of the high pressure regulators ability to supply fuel at higher flow rates.

    5.

  5. This step checks for a restricted high pressure regulator (HPR) filter. If a restricted HPR filter is found also inspect the fill valve filter for contamination, damage or restriction.

    6.

  6. This step checks the ability of the AF ECU to energize the high pressure lock-off (HPL) solenoid.

    7.

  7. This step checks for a closed manual lock-down screw.

    8.

  8. This step checks for the sound of a functioning HPL.

    9.

  9. This step checks for damage to the fuel line between the HPL and the HPR.

    10.

  10. This step checks the ability of the AF ECU to energize the low pressure lock-off (LPL) solenoid.

    11.

  11. This step checks for the sound of a functioning LPL.

    12.

  12. This step checks for damage to the fuel line between the HPR and the LPR.

    13.

  13. This step checks for proper operation of the system after repairs are complete.

Step

Action

Value(s)

Yes

No

1

Perform the AF Powertrain On-Board Diagnostic (OBD) System Check.

Are any AF ECU DTCs except P1202 set?

--

Go to applicable DTC diagnosis. Go to the 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.

Does the vehicle have adequate CNG fuel pressure?

8274 kPa (1200 psi)

Go to Step 3

--

3

  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and return to this step.
  2. Turn the ignition OFF.
  3. Remove the pressure relief device (PRD) from the high pressure regulator (HPR).
  4. Install the J 39970-UPD Electronic Pressure Gauge Adaptor into the PRD port of the HPR.
  5. Connect the J 39970 Electronic Pressure Gauge to the J 39970-UPD
  6. Connect the J 39200 DMM to the J 39970 .
  7. Observe the pressure while cranking the engine.

    8. If a DTC sets that forces a switch-over (such as DTC P1202) the fuel pressure may only be observed up until the DTC sets. In the instances when the engine will not idle on CNG use the pressure values observed during cranking and proceed directly to the Step question.

  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. Observe and record the pressure indicated by the DMM during engine load (brake-torque) by performing the following steps:
    11. 10.1. Apply the parking brake.
    10.2. Firmly depress and hold the brake pedal.
    10.3. Place the vehicle in forward gear.
    10.4. Observe the pressure while depressing the throttle to 80 percent or greater for a maximum of 10 seconds.
    10.5. 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.

Were all pressure values within the specified pressure range?

130-220 psi

Go to Step 5

Go to Step 4

4

Was the pressure greater than the specified pressure range?

220 psi

Go to Step 19

Go to Step 10

5

  1. Turn the ignition OFF.

    2. Leave the J 39970 Electronic Pressure Gauge connected to the HPR.

  2. Remove the low pressure regulator (LPR) Primary test port plug.
  3. Insert the pressure gauge adaptor included in the J 42580 Low Pressure Test Kit into the LPR Primary test port.
  4. Connect the J 28552 0-15 psi gauge to the pressure gauge adaptor.
  5. Observe the pressure while cranking the engine.

    6. If a DTC sets that forces a switch-over (such as DTC P1202) the fuel pressure may only be observed up until the DTC sets. In the instances when the engine will not idle on CNG use the pressure values observed during cranking and proceed directly to the Step question.

  6. Idle the engine until normal operating temperature is obtained.
  7. Observe and record the pressure indicated by the gauge at engine idle.
  8. Observe and record the pressure indicated by the gauge during engine load (brake-torque) by performing the following steps:
    9. 8.1. Apply the parking brake.
    8.2. Firmly depress and hold the brake pedal.
    8.3. Place the vehicle in forward gear.
    8.4. Observe the pressure while depressing the throttle to 80 percent or greater for a maximum of 10 seconds.
    8.5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  9. Compare the recorded pressure values to the value range specified.

Were all pressure values within the specified pressure range?

4-6 psi

Go to Step 7

Go to Step 6

6

Was the pressure greater than the specified pressure range?

6 psi

Go to Step 18

Go to Step 15

7

  1. Turn the ignition OFF.
  2. Remove the low pressure regulator (LPR) SECONDARY test port plug.

    3. A small amount of oil may drain from this port. Refer to Test Descriptions .

  3. Transfer the pressure gauge adaptor from the LPR Primary test port into the SECONDARY test port.
  4. Re-install the LPR primary test port plug using Teflon sealant. DO NOT use Teflon tape.
  5. Connect the J 7333-6 Slack Tube Manometer to the pressure gauge adaptor.

    6. Important: Keep the vent port of the manometer CLOSED until AFTER the engine is started.

  6. Start and idle the engine until normal operating temperature is obtained.
  7. Observe and record the pressure indicated by the manometer at engine idle.
  8. Observe and record the pressure indicated by the manometer during engine load (brake-torque) by performing the following steps:
    9. 8.1. Apply the parking brake.
    8.2. Firmly depress and hold the brake pedal.
    8.3. Place the vehicle in forward gear.
    8.4. Observe the pressure while depressing the throttle to 80 percent or greater for a maximum of 10 seconds.
    8.5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  9. Compare the recorded pressure values to the value range specified.

Were both values within the specified pressure range?

4.5-7 inches of water column

Go to Step 9

Go to Step 8

8
  1. Inspect the vapor hose between the LPR and the gas mass sensor/mixture control valve (GMS/MCV) and the vapor hose between the GMS/MCV and the gas distribution adaptor (GDA)
  2. Check for loose clamps, cracks or deterioration.
  3. Inspect the GDA for physical damage and loose or missing components.
  4. Inspect the butterfly valve located within the bore of the GMS/MCV.
  5. Check that the butterfly rotates freely and does not drag when rotated.
  6. Repair or replace components as necessary.

Was a repair or replacement performed?

--

Go to Step 22

Go to Step 18

9

  1. Turn the ignition OFF.
  2. Remove the manometer and the pressure gauge adaptor from the low pressure regulator (LPR) secondary test port.
  3. Re-install the LPR secondary test port plug using Teflon sealant. DO NOT use Teflon tape.
  4. Place the J 39200 DMM (connected to the J 39970 Electronic Pressure Gauge in step 3) in the vehicle in order to perform a road-load test of the High Pressure Regulator.

    5. Ensure all testing equipment and wiring is correctly routed for on-road operation of the vehicle.

    Caution: Test drive the vehicle in a remote area away from traffic with a passenger watching the gauge in order to prevent personal injury.

  5. Test drive the vehicle with a passenger recording the DMM pressure values using the following procedure:

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

    5.1. Record the pressure with the vehicle stopped and in gear.
    5.2. Depress the throttle to at least 80 percent allowing the vehicle to accelerate. Observe and record the pressure as the vehicle accelerates.

    Do not exceed posted speed limits.

  6. Compare the recorded pressure values to the specified pressure range.

Was the pressure within the specified pressure range?

130-220 psi

Go to Step 22

Go to Step 19

10

  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and returning to this step.
  2. Replace the high pressure regulator (HPR) filter. Refer to High Pressure Regulator Filter Replacement .
  3. Observe the pressure while cranking the engine.

    4. If a DTC sets that forces a switch-over (such as DTC P1202) the fuel pressure may only be observed up until the DTC sets. In the instances when the engine will not idle on CNG use the pressure values observed during cranking and proceed directly to the Step question.

  4. Idle the engine until normal operating temperature is obtained.

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

  5. Observe and record the pressure indicated by the DMM at engine idle.
  6. Observe and record the pressure indicated by the DMM during engine load (brake-torque) by performing the following steps:
    7. 6.1. Apply the parking brake.
    6.2. Firmly depress and hold the brake pedal.
    6.3. Place the vehicle in forward gear.
    6.4. Observe the pressure while depressing the throttle to 80 percent or greater for a maximum of 10 seconds.
    6.5. Allow the engine to idle for a minimum of two minutes in order to cool the transmission.
  7. Compare the recorded pressure values to the value range specified.

Were all pressure values within the specified pressure range?

130-220 psi

Go to Step 22

Go to Step 11

11

  1. Turn the ignition OFF.
  2. Remove the fuel tank cover. Refer to CNG Fuel Tank Cover Replacement .
  3. Disconnect the high pressure lock-off (HPL) solenoid connector (C313).
  4. Connect the J 35616-200 Un-powered Test Lamp between the HPL connector terminals (harness side).
  5. While observing the test lamp crank the engine.

Did the test lamp illuminate?

--

Go to Step 12

Go to Lock-Off Relay Diagnosis

12

  1. Turn the ignition OFF.
  2. Verify that the manual lock-down screw of the HPL is fully open. The Allen screw should be fully seated counter-clockwise.
  3. Open the manual lock-down screw if necessary.

Was the manual lock-down screw fully open?

--

Go to Step 13

Go to Step 22

13

  1. Turn the ignition OFF.
  2. Connect the HPL connector.
  3. The AF ECU will energize the HPL for about one second at ignition ON.

    4. Listen and/or feel for HPL operation as the ignition is turned ON.

Was HPL operation heard or felt as the ignition was turned ON?

--

Go to Step 14

Go to Step 20

14

  1. Inspect the fuel line between the HPL and the HPR for the following:
    2. • kinks
    • bends
    • damage
    • obstruction
    • An obstructed fuel line will be cool or frosted near the obstruction when fuel flows through the line.
  2. Repair or replace components as necessary.

Was a repair or replacement made?

--

Go to Step 22

Go to Step 19

15

  1. Turn the ignition OFF.
  2. Disconnect the Low Pressure Lock-Off (LPL) solenoid connector (C014).
  3. Connect the J 35616-200 Un-powered Test Lamp between the LPL connector terminals (harness side).
  4. While observing the test lamp crank the engine.

Did the test lamp illuminate while the engine was cranking?

--

Go to Step 16

Go to Lock-Off Relay Diagnosis

16

  1. Connect a jumper harness between the LPL solenoid connector terminal A and battery positive.
  2. Momentarily connect a jumper harness between the LPL solenoid connector terminal B and ground.
  3. Listen and/or feel for LPL operation as you energize and de-energize the LPL solenoid using the jumper harness.

Was LPL operation heard or felt when energized?

--

Go to Step 17

Go to Step 21

17

  1. Inspect the fuel line between the LPL and the HPR for the following:
    2. • kinks
    • bends
    • damage
    • obstruction
  2. Repair or replace components as necessary.

Was a repair or replacement made?

--

Go to Step 22

Go to Step 18

18

Replace the low pressure regulator (LPR). Refer to Compressed Natural Gas Low Pressure Regulator and Bracket Replacement .

Is the replacement complete?

--

Go to Step 22

--

19

Important: If you were sent to this step because the observe fuel pressure was too low ensure that the fuel line between the HPL and the HPR is unobstructed prior to replacing the HPR.

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

Is the replacement complete?

--

Go to Step 22

--

20

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

Is the replacement complete?

--

Go to Step 22

--

21

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

Is the replacement complete?

--

Go to Step 22

--

22

  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and returning to this step.
  2. Turn the ignition OFF.
  3. Remove all remaining test equipment.
  4. Ensure the pressure relief device (PRD) is re-installed into the high pressure regulator test port using Teflon sealant. DO NOT use Teflon tape.
  5. Ensure the test port plugs are re-installed into the low pressure regulator test ports using Teflon sealant. DO NOT use Teflon tape.
  6. Start and idle the engine.
  7. Inspect for CNG fuel system leaks between the fuel tank and the LPR using the J 41416 Ultrasonic Leak Detector.
  8. Inspect for CNG fuel system leaks at the LPR and between the LPR and the gas distribution ring (GDR) using Snoop® or soapy water.
  9. Repair any leaks as necessary.
  10. Using a scan tool clear DTC Information from the AF ECU and from the gasoline Control Module (PCM, VCM or ECM) as necessary.
  11. Turn the ignition OFF and wait 30 seconds.
  12. Start the engine and operate the vehicle.
  13. Observe the fuel indicator lamp and the MIL.
  14. Observe vehicle performance and driveability.

Does the vehicle operate on CNG with normal driveability and no MIL illumination?

--

System OK

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check