GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Diagnosis

LPR Primary Test Port
Object Number: 327049 Size: SF
(1)Primary Test Port

LPR Secondary Test Port
Object Number: 327051 Size: SF
(1)Secondary Test Port

HPR Test Port
Object Number: 327053 Size: SF

The vehicle will start and operate on CNG if there is sufficient CNG in the fuel tank. A high pressure lock-off (HPL) solenoid in the tank prevents fuel flow unless the engine is cranking or running. CNG flows from the tank through the high pressure fuel lines at up to 24 820 kPa (3,600 psi) to the high pressure regulator (HPR) when the key is ON and the engine is running. The pressure is reduced to approximately 1 172 kPa (170 psi) at the high pressure regulator (HPR). CNG continues to flow through the medium pressure fuel lines to the low pressure lock-off (LPL) solenoid. The LPL also prevents fuel flow unless the engine is cranking or running. Fuel flows from the LPL to the low pressure regulator (LPR) where the pressure is reduced to approximately 1.5 kPa (0.21 psi). Fuel then flows to the gas mass sensor/mixture control valve (GMS/MCV). The GMS/MCV measures and controls the amount of CNG delivered to the gas distribution adapter (GDA). The GDA introduces the CNG to the air intake stream above the throttle body. This air/fuel mixture is then drawn into the engine for combustion.

Diagnostic Aids

Tools Required

    • J 7333-6 Slack Tube Manometer
    • J 28552 0-15 PSI Gauge
    • J 39200 High Impedance Multimeter
    • J 39970 CNG Electronic Pressure Gauge
    • J 39970-UPD Electronic Pressure Gauge Adapter
    • J 42580 Low Pressure Test Kit

Test Description

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

  1. This step determines if the LPR is accurately regulating primary pressure and if the malfunction is upstream or downstream of the LPR.

    2.

  2. If the primary pressure is greater than 6 psi, the primary stage of the LPR is not correctly regulating pressure.

    3.

  3. This step determines if the resistance of the LPL coil is within specification. An LPL coil out of specification may not open properly and cause a fuel flow restricition.

    4.

  4. Low LPR secondary pressure may indicate a leak in the AF fuel system after the LPR. Inspect for damaged or leaking low pressure fuel hoses or a damaged or leaking GDA.

    Important: The J 41416 Ultrasonic Leak Detector will not detect a leak past the LPR outlet fitting.

  5. A test drive is required to verify that the HPR will maintain adequate fuel pressure under maximum load.

    6.

  6. This step determines if the resistance of the HPL coil within specification. An HPL coil out of specification may not open properly and cause a fuel flow restriction.

Step

Action

Value(s)

Yes

No

1

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

Are any DTCs present in the AF ECU?

--

Go to the applicable DTC Table

Go to Step 2

2

Verify that the CNG fuel tank has a minimum of 1/4 tank of fuel.

Does the vehicle have fuel?

--

Go to Step 3

--

3

  1. Connect the J 28552 0-15 PSI Gauge using the J 42580 Low Pressure Test Kit in the primary test port of the low pressure regulator (LPR).
  2. Start the engine. Allow it to reach operating temperature.
  3. Record the pressure reading at idle.
  4. Apply the parking brake.
  5. Place the vehicle in gear. Firmly hold the brake pedal and open the throttle to 80% or greater. Observe the pressure gauge.
  6. Record the minimum readings obtained.
  7. Allow the engine to idle for two minutes.

Was the LPR primary pressure within the specified range?

4-6 psi

Go to Step 9

Go to Step 4

4

Was the LPR primary pressure greater than the specified value?

6 psi

Go to Step 20

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the low pressure lock-off (LPL) connector.
  3. Install a test light between the pins of the LPL connector.
  4. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 6

Go to Step 8

6

Verify the resistance of the LPL using a DVOM.

Is the resistance within the specified range?

6.2-10 ohms

Go to Step 7

Go to Step 21

7
  1. Close the CNG manual shut-off valve (MSV).
  2. Reconnect the LPL connector.
  3. Start the engine.
  4. Turn OFF the ignition after the vehicle switches to gasoline or stalls.

    5. Important:: This procedure may set a DTC 51 (GMS frequency low) in the AF system.

  5. Remove the pressure relief device (PRD) from the high pressure regulator (HPR).
  6. Connect the J 39970 CNG Electronic Pressure Gauge using the J 39970-UPD Electronic Pressure Gauge Adapter in the PRD port of the HPR.
  7. Open the CNG MSV.
  8. Start the engine. Allow it to reach operating temperature.
  9. Record the pressure reading at idle.
  10. Apply the parking brake.
  11. Place the vehicle in gear. Firmly hold the brake pedal and open the throttle to 80% or greater. Observe the pressure gauge.

    12. Important:: Rhythmic fluctuations of 10-20 psi in pressure are normal. They do not indicate a defective HPR unless the fluctuations become excessive.

  12. Record the minimum and maximum readings obtained.
  13. Allow the engine to idle for two minutes.

Was the HPR pressure within the specified range?

130-220 psi

Go to Step 20

Go to Step 12

8
  1. Turn OFF the ignition.
  2. Connect the test light to chassis ground.
  3. Probe pin A of the LPL connector.
  4. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 18

Go to Step 19

9
  1. Turn OFF the engine.
  2. Connect the J 7333-6 Slack Tube Manometer using the J 42580 in the secondary test port of the low pressure regulator (LPR).
  3. Start the engine. Allow it to reach operating temperature.

    4. Important:: Keep the vent port of the water column gauge closed until after the engine is started.

  4. Record the pressure reading at idle.
  5. Apply the parking brake.
  6. Place the vehicle in gear. Firmly hold the brake pedal and open the throttle to 80% or greater. Observe the pressure gauge.

    7. Important:: Rhythmic fluctuations of 10-20 psi in pressure are normal. They do not indicate a defective HPR unless the fluctuations become excessive.

  7. Record the minimum and maximum readings obtained.
  8. Allow the engine to idle for two minutes.

Was the LPR secondary pressure within the specified range?

4.5"-7" w.c.

Go to Step 11

Go to Step 10

10

  1. Inspect the vapor hose between the LPR and the GMS/MCV for loose clamps, cracks or deterioration.
  2. Inspect the vapor hose between the GMS/MCV and the gas distribution adapter (GDA) for loose clamps, cracks or deterioration.
  3. Remove and inspect the GDA for physical damage and loose or missing components. Refer to Gas Distribution Adapter Replacement .
  4. Inspect the butterfly in the bore of the GMS/MCV. Verify that the butterfly moves freely and does not drag when rotated.

Was a problem found?

--

Go to Step 22

Go to Step 20

11

  1. Close the CNG manual shut-off valve (MSV).
  2. Turn OFF the ignition after the vehicle switches to gasoline or stalls.

    3. Important:: This procedure may set a DTC 51 (GMS Frequency Low).

  3. Remove the pressure relief device (PRD) from the high pressure regulator (HPR).
  4. Install the J 39970 using the J 39970-UPD in the PRD port of the HPR.
  5. Open the CNG MSV.
  6. Start the engine. Allow it to reach operating temperature.
  7. Record the pressure reading at idle.

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

  8. Test drive the vehicle with a passenger watching the gauge. From a stop, depress the throttle to 80% or greater with the passenger noting the pressure gauge reading.

    9. Important:: Rhythmic fluctuations of 10-20 psi in pressure are normal. They do not indicate a defective HPR unless the fluctuations become excessive.

  9. Record the minimum and maximum readings during the road test.

Were all the HPR pressure readings within the specified range?

130-220 psi

Go to Step 24

Go to Step 12

12

Was the HPR pressure greater than the specified value?

220 psi

Go to Step 23

Go to Step 13

13
  1. Close the CNG manual shut-off valve (MSV).
  2. Replace the HPR filter. Refer to High Pressure Regulator Filter Replacement .
  3. Open the CNG MSV.
  4. Start the engine. Allow it to reach operating temperature.
  5. Record the pressure reading at idle.
  6. Apply the parking brake.
  7. Place the vehicle in gear. Firmly hold the brake pedal and open the throttle to 80% or greater. Observe the pressure gauge.

    8. Important:: Rhythmic fluctuations of 10-20 psi in pressure are normal. They do not indicate a defective HPR unless the fluctuations become excessive.

  8. Record the minimum and maximum readings obtained.
  9. Allow the engine to idle for two minutes.

Was the HPR pressure within the specified range?

130-220 psi

Go to Step 24

Go to Step 14

14

Inspect for the following conditions:

    • Kinks, bends or other damage to the CNG fuel lines.
    • Obstructions (frosting or cool to touch during running) in the CNG fuel lines.
    • Partially closed CNG manual shutoff valve (MSV).
    • Verify that the manual lock down screw in the HPL is fully open.

Was a problem found?

--

Go to Step 22

Go to Step 15

15
  1. Turn OFF the ignition.
  2. Disconnect the HPL connector.
  3. Install a test light between the pins of the HPL connector.
  4. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 16

Go to Step 17

16

Verify the resistance of the HPL using a DVOM.

Is the resistance within the specified range?

16.3-20.6 ohms

Go to Step 23

Go to Step 21

17
  1. Turn OFF the ignition.
  2. Connect the test light to chassis ground.
  3. Probe pin A of the HPL connector.
  4. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 18

Go to Step 19

18

Repair the open in the lock-off ground circuit. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual.

Is the repair complete?

--

Go to Step 24

--

19

Repair the open in the lock-off power circuit. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual.

Is the repair complete?

--

Go to Step 24

--

20

Replace the LPR. Refer to Low Pressure Regulator and Mounting Bracket Replacement .

Is the repair complete?

--

Go to Step 24

--

21

Replace the lock-off. Refer to Low Pressure Lock-Off Solenoid Replacement or High Pressure Lock-Off Solenoid Replacement .

Is the repair complete?

--

Go to Step 24

--

22

Replace the component as necessary.

Is the repair complete?

--

Go to Step 24

--

23

Replace the HPR. Refer to High Pressure Regulator and Mounting Bracket Replacement .

Is the repair complete?

--

Go to Step 24

--

24

  1. Close the CNG manual shut-off valve (MSV).
  2. Start and run the engine until it stalls.
  3. Disconnect all test equipment.
  4. Install the primary and secondary test port plugs and the PRD.
  5. Start the engine.
  6. Leak check the test port plugs and the PRD using the J 41416 Ultrasonic Leak Detector or the equivalent.

Is the action complete?

--

System OK

--