GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Diagnosis

System Description

The alternative fuel engine control module (ECM) controls the fuel system while the vehicle is operating on alternative fuel. 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 cylindrical fuel tanks. A full tank can contain fuel at 24,820 kPa (3,600 psi). A high pressure lock-off (HPL) solenoid in the tank and a low pressure lock-off (LPL) solenoid in the fuel line both allow and prevent fuel flow. The Alternative Fuels ECM commands ONLY the HPLs open for about 2 seconds at every ignition cycle 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 tanks. The Alternative Fuels ECM commands BOTH the HPLs 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 below atmospheric pressure.

High Pressure - Tank - Stage

When the Alternative Fuels ECM commands the HPL open fuel begins to flow, at up to 3,600 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 130-220 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 Alternative Fuels ECM. Fuel flows out of the LPL and into the low pressure regulator (LPR). The inlet of the LPR is the Low Pressure Stage.

Low Pressure Stage

The low pressure regulator (LPR) is a 2-stage regulator. The fuel enters the LPR at the intermediate stage pressure of 130-220 psi. The primary stage of LPR operation reduces the fuel pressure down to about 5 psi. The fuel pressure is then again reduced in the secondary stage of the LPR down to 0.5 inches of water column. The fuel exits the LPR and flows through the low pressure hose and into the fuel mixer.

Fuel Control Solenoid (FCS)

The FCS is an electronically controlled solenoid used to trim fuel mixtures by metering air valve vacuum to control the pressure on top of the LPR secondary diaphragm. This changes the LPR output pressure, and the amount of fuel flowing out of the LPR. The Alternative Fuels ECM controls the FCS with an electrical duty cycle which is converted into the regulated air valve vacuum signal communicated to the top of the LPR. As the FCS duty cycle increases, more vacuum on top of the LPR secondary diaphragm reduces the fuel flow out of the LPR which leans the air fuel mixture. As the FCS duty cycle decreases, less vacuum on top of the LPR secondary diaphragm increases the fuel flow out of the LPR, enriching the air fuel mixture. The fuel control authority of the FCS increases with engine air flow.

Idle Control Solenoid (ICS)

The ICS is an electronically controlled solenoid used to meter supplemental fuel from the LPR to the mixer to increase fuel control authority during idle and low speed operation. The Alternative Fuels ECM controls the ICS with an electrical duty cycle which is converted into the regulated fuel flow from the LPR to the mixer. As the ICS duty cycle increases, more supplemental fuel flows out of the LPR into the mixer, enriching the air fuel mixture. As the ICS duty cycle decreases, less supplemental fuel flows out of the LPR into the mixer which leans the air fuel mixture. The fuel control authority of the ICS decreases with engine air flow.

Diagnostic Aids

The fuel control solenoid (FCS) and idle control solenoid (ICS) connectors are inter-changeable. Ensure that the RED connector is connected to the ICS. The ICS is the most rearward solenoid.

The low pressure regulator (LPR) reference hose connects the LPR to the air cleaner outlet duct. This easily dislodged hose can cause driveability concerns if disconnected or restricted.

Test Description

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

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

    2.

  2. This step tests 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. This condition is normal and no repair is required.

    This step tests the fuel pressure output of the secondary stage of the low pressure regulator. The engine must be flowing the desired amount of airflow in order to accurately observe the secondary fuel pressure.

    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 inspects 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 tests the ability of the Alternative Fuels ECM to energize the high pressure lock-off (HPL) solenoid.

    7.

  7. This step inspects for closed manual lock-down screws.

    8.

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

    9.

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

    10.

  10. This step tests the ability of the Alternative Fuels ECM to energize the low pressure lock-off (LPL) solenoid.

    11.

  11. This step inspects 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 inspects for proper operation of the system after repairs are complete.

Step

Action

Value(s)

Yes

No

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 the CNG fuel tank if necessary.

Does the vehicle have adequate CNG fuel pressure?

8274 kPa (1,200 psi)

Go to Step 3

--

3

  1. Remove the engine cover. Refer to Engine Cover Replacement in Interior Trim in the G Van Service Manual.
  2. Relieve fuel system pressure. Refer to Fuel Pressure Relief .
  3. Turn OFF the ignition.
  4. Connect the J 39970 Electronic Pressure Gage to the J 39970-100 Electronic Pressure Gage Adaptor.
  5. Install the J 39970-100 into the CNG fuel line test port at the rear of the engine.
  6. Connect a digital multimeter (DMM) to the J 39970 .
  7. Zero the J 39970 as directed by the tool documentation. Ensure the CNG fuel line is not pressurized when zero-ing the J 39970 .

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

  8. Observe the pressure while cranking the engine.

    9. If a DTC sets that forces a switch-over, 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.

  9. Idle the engine until normal operating temperature is obtained.
  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 depress and hold the brake pedal.
    11.3. Place the vehicle in forward gear.
    11.4. Observe the pressure while depressing the throttle to 80 percent or more for a maximum of 10 seconds.
    11.5. Allow the engine to idle for a minimum of 2 minutes in order to cool the transmission.
  12. Compare the recorded pressure values to the value range specified.

Were all pressure values within the specified range?

130-220 psi

Go to Step 5

Go to Step 4

4

Is the pressure more than the specified value?

220 psi

Go to Step 23

Go to Step 14

5

  1. Turn OFF the ignition.

    2. Leave the J 39970 connected to the fuel line test port.

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

    6. If a DTC sets that forces a switch-over, 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 gage at engine idle.
  8. Compare the recorded pressure value to the value range specified.

Is the pressure value within the specified range?

4-6 psi

Go to Step 7

Go to Step 6

6

Is the pressure more than the specified value?

6 psi

Go to Step 22

Go to Step 19

7

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

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

  3. Transfer the pressure gage 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 gage adaptor ensuring that the vent port is closed.
  6. Disconnect the idle control solenoid (ICS) and fuel control solenoid (FCS) electrical connectors. A DTC will set.

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

  7. Start and idle the engine until normal operating temperature is obtained.
  8. Open the vent port on the J 7333-6 .
  9. Observe the mass air flow scan tool data parameter while you increase the throttle opening to obtain the specified engine airflow.
  10. Observe and record the pressure indicated by the manometer when the engine is operating at the specified air flow.
  11. Compare the recorded pressure value to the value range specified.

Is the value within the specified range?

42 gm/s

0.25-0.65  negative inches of water column

Go to Step 8

Go to Step 12

8

Reconnect the idle control solenoid (ICS) while observing the manometer pressure. The ICS connector is RED and the ICS is the rearward solenoid.

Did the manometer pressure change slightly?

--

Go to Step 9

Go to Step 10

9

Reconnect the fuel control solenoid (FCS) while observing the manometer pressure.

Did the manometer pressure change slightly?

--

Go to Step 13

Go to Step 11

10
  1. Inspect the lines between the idle control solenoid and the low pressure regulator for the following:
    2. • Kinks
    • Leaks
    • Damage
    • Obstructions
  2. Repair or replace components as necessary.

Was a repair or replacement made?

--

Go to Step 28

Go to Step 26

11
  1. Inspect the lines between the fuel control solenoid and the low pressure regulator for the following:
    2. • Kinks
    • Leaks
    • Damage
    • Obstructions
  2. Repair or replace components as necessary.

Was a repair or replacement made?

--

Go to Step 28

Go to Step 27

12
  1. Inspect the hose between the low pressure regulator and the fuel mixer for loose clamps, cracks, or deterioration.
  2. Inspect the throttle body adapter for physical damage and loose or missing components.
  3. Inspect that the fuel mixer moves freely and does not drag. Refer to Fuel Mixer Replacement .
  4. Repair or replace components as necessary.

Was a repair or replacement performed?

--

Go to Step 28

Go to Step 22

13

  1. Turn OFF the ignition.
  2. Remove the manometer and the pressure gage 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 DMM, connected to the J 39970 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 high pressure regulator 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.

Is the pressure within the specified range?

130-220 psi

Go to Step 28

Go to Step 23

14

  1. Relieve fuel system pressure by performing the Fuel Pressure Relief and return 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, 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 high pressure regulator 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 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 more for a maximum of 10 seconds.
    6.5. Allow the engine to idle for a minimum of 2 minutes in order to cool the transmission.

Were all pressure values within the specified range?

130-220 psi

Go to Step 28

Go to Step 15

15

  1. Turn OFF the ignition.
  2. Remove the fuel tank shield. Refer to Fuel Tank Shield Replacement .
  3. Disconnect a high pressure lock-off (HPL) solenoid connector.
  4. Connect the J 35616-200 Unpowered Test Lamp between the high pressure lock-off solenoid connector terminals - vehicle harness side.
  5. While observing the test lamp, crank the engine.

Does the test lamp illuminate?

--

Go to Step 16

Go to Fuel Lock-Off Relay Diagnosis

16

  1. Turn OFF the ignition.
  2. Verify that the manual lock-down screws of the high pressure lock-offs are fully open. The Allen screw should be fully seated counterclockwise.
  3. Open the manual lock-down screws if necessary.

Were the manual lock-down screws fully open?

--

Go to Step 17

Go to Step 28

17

  1. Turn OFF the ignition.
  2. Connect the high pressure lock-off solenoid connector.
  3. The Alternative Fuels ECM will energize the high pressure lock-off relay for about 1 second at ignition ON.

    4. Listen and/or feel for high pressure lock-off (HPL) solenoid operation as the ignition is turned ON.

Is HPL operation heard or felt as the ignition is turned ON?

--

Go to Step 18

Go to Step 24

18

  1. Inspect the fuel line between the high pressure lock-off solenoid and the high pressure regulator for the following:
    2. • Kinks
    • Bends
    • Damage
    • Obstructions
    • 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 28

Go to Step 23

19

  1. Turn OFF the ignition.
  2. Disconnect the low pressure lock-off (LPL) solenoid connector.
  3. Connect the J 35616-200 between the LPL connector terminals - harness side.
  4. While observing the test lamp, crank the engine.

Does the test lamp illuminate while the engine is cranking?

--

Go to Step 20

Go to DTC P1215

20

  1. Start and idle the engine.
  2. Command the low pressure lock-off solenoid ON and OFF using the scan tool alternative fuels special functions.
  3. Listen and/or feel for LPL operation as you energize and de-energize the LPL solenoid.

Is LPL operation heard or felt when energized?

--

Go to Step 21

Go to Step 25

21

  1. Inspect the fuel line between the low pressure lock-off solenoid and the high pressure regulator for the following:
    2. • Kinks
    • Bends
    • Damage
    • Obstructions
  2. Repair or replace components as necessary.

Was a repair or replacement made?

--

Go to Step 28

Go to Step 22

22

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

Is the replacement complete?

--

Go to Step 28

--

23

Important: If you were sent to this step because the observed fuel pressure was too low, ensure that the fuel line between the high pressure lock-off and the high pressure regulator is unobstructed prior to replacing the high pressure regulator.

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

Is the replacement complete?

--

Go to Step 28

--

24

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

Is the replacement complete?

--

Go to Step 28

--

25

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

Is the replacement complete?

--

Go to Step 28

--

26

Replace the idle control solenoid (ICS). Refer to Idle Control Solenoid Replacement .

Is the replacement complete?

--

Go to Step 28

--

27

Replace the fuel control solenoid (FCS). Refer to Fuel Control Solenoid Replacement .

Is the replacement complete?

--

Go to Step 28

--

28

  1. Turn OFF the ignition.
  2. Remove all remaining test equipment.
  3. Ensure the test port plugs are re-installed into the low pressure regulator test ports using Teflon sealant. DO NOT use Teflon tape.
  4. Start and idle the engine.
  5. Inspect for CNG fuel system leaks between the fuel tank and the low pressure regulator using the J 41416 Ultrasonic Leak Detector.
  6. Inspect for CNG fuel system leaks at the low pressure regulator using Snoop® or soapy water.
  7. Repair any leaks as necessary.
  8. Using a scan tool, clear DTC information from the Alternative Fuels ECM and from the gasoline control module (PCM, VCM or ECM) as necessary.
  9. Turn OFF the ignition and wait 30 seconds.
  10. Start the engine and operate the vehicle.
  11. Observe the fuel indicator lamp and 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)