GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Unwanted Switch to Gasoline

Circuit Description

The AF ECU and the gasoline control module (VCM/PCM or ECM) are connected by the Natural Gas Operation (NGO) enable circuit. In order for the engine to switch-over to CNG operation the gasoline control module must be commanded to disable the gasoline fuel injectors, switch to a different ignition spark calibration and disable some of the gasoline control module DTCs. The AF ECU performs the switch-over command by grounding the NGO enable circuit when CNG operation is desired.

This vehicles primary fuel source is Compressed Natural Gas (CNG). The vehicle will only switch-over to gasoline operation if one of the following conditions is present:

    • The fuel pressure sensor (FPS) indicates the CNG tank is empty.
    • A wide-open Mixture Control Valve (MCV) is detected.
    • A system fault is detected by the AF ECU.
    • Engine crank time exceeds about 8 seconds.
    • Every 100th start and also when the coolant temperature is below 10°F (-12°C), the vehicle will start and run briefly on gasoline. A switch-over to CNG will occur only during a decel condition.

Diagnostic Aids

This diagnostic information should be utilized only if the concern of the vehicle operator can be duplicated. An unwanted switch may occur only during specific vehicle operation. An example follows:

  1. The vehicle has an intermittently leaking low pressure lock-off (LPL) solenoid.
  2. After turning the engine OFF and waiting several minutes all of the CNG purges from the lines.
  3. During the next ignition ON key cycle the high pressure lock-off (HPL) is opened for about 1 second.

    4. At the same moment that the HPL is opened the AF ECU monitors the fuel pressure sensor.

    If the LPL is still leaking the CNG pressure cannot build within the system and low fuel pressure is observed by the AF ECU.

    Detecting the low fuel pressure in the system the AF ECU performs a switch-over to gasoline.

  4. The vehicle may start and run on CNG during any ignition cycle in which the LPL does not leak.

    5. The customer concern would be an intermittent unwanted switch to gasoline with no DTCs stored.

Fault Not Present

Fault Not Present indicates the condition that caused the concern is intermittent and not currently present. Refer to Intermittent Conditions symptom diagnosis for additional information. If any additional concerns were observed return to the AF Powertrain OBD System Check.

Test Description

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

  1. The AF Powertrain On-Board Diagnostic (OBD) System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the OBD System Check will result in mis-diagnosis.

    2.

  2. This step checks if the AF ECU has commanded the gasoline control module (using the NGO Enable circuit) to start on CNG.

    The NGO Enable Output parameter is the commanded state of the NGO Enable circuit from the AF ECU. The NGO Enable Input parameter is the monitored state of the NGO Enable circuit from the GASOLINE control module (VCM/PCM or ECM).

    3.

  3. This step checks if the FIL is illuminated during CNG operation. The FIL should illuminate only during gasoline operation.

    4.

  4. This step checks if the FPS is monitoring adequate fuel tank pressure for the vehicle to start on CNG.

    5.

  5. This step checks the ability of the AF ECU to command Natural Gas Operation (NGO) and the gasoline control modules ability to monitor the command.

    6.

  6. This step indicates all the possibilities that cause a switch-over to gasoline operation.

    7.

  7. This step checks the ignition system of the vehicle for proper operation. A vehicle that operates correctly on gasoline but not on CNG, may have a faulty ignition system.

    8.

  8. Performing the AF Fuel System Diagnosis checks the basic AF fuel system for proper pressures and lock-off solenoid operation.

    9.

  9. A fuel system that is unable to retain fuel pressure after the ignition is turned OFF, may cause an Unwanted Switch to Gasoline. The AF ECU commands the HPL open for about 1 second every time the ignition is turned ON. If the fuel line system leaks during ignition OFF the next ignition ON prime pulse will usually recharge the fuel lines and the FPS. A sluggish HPL may not open during the 1 second prime pulse but will open when commanded with the scan tool.

    10.

  10. This step checks for proper fuel pressure sensor operation.

    11.

  11. This step checks for an inoperative HPL. After connecting to a CNG dispensing station the CNG fuel lines, the FPS and the fuel tank will be charged by the station. The CNG fuel pressure in the lines and the FPS will quickly drop if the engine is operating on CNG but the HPL fails to open.

    12.

  12. This step checks for proper operation of the NGO Enable circuit and the gasoline control module.

    13.

  13. This step checks for a leaking fuel system with a sluggish HPL.

Step

Action

Value(s)

Yes

No

1

Were you sent here from the AF Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

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

2

  1. Using the scan tool, communicate with the AF ECU and observe the NGO Enable Input and Output data parameters.
  2. Observe the data parameters as you crank and start the engine.

Do the data parameters indicate that Natural Gas Operation is enabled?

--

Go to Step 3

Go to Step 5

3

Observe the fuel indicator lamp (FIL) located in the fuel gauge select switch button with the vehicle running.

Is the FIL illuminated?

--

Go to Fuel Gauge Selector Switch/Fuel Indicator Lamp Diagnosis

Go to Step 4

4

The fault is not present. Refer to Diagnostic Aids.

Is their a system operation concern?

--

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

System OK

5

Using the scan tool communicate with the AF ECU and observe the Fuel Pressure Sensor data parameter.

Does the Fuel Pressure Sensor indicate pressure more than the value specified?

8274 kPa (1200 psi)

Go to Step 6

Go to Step 10

6

  1. Command the NGO Enable circuit to natural gas operation using the scan tool.
  2. Observe the NGO Enable Input data parameter.

Does the parameter indicate that Natural Gas Operation is enabled?

--

Go to Step 7

Go to Step 13

7

  1. Natural gas operation will be disabled when any of the following conditions are met:
    2. • The fuel pressure sensor (FPS) indicates fuel pressure drops below 0.85 volts (2412 kPa @ 21°C [350 psi @ 70°F]).
    • Natural Gas Operation will NOT be enabled until the fuel pressure sensor indicates fuel pressure is greater than 1.50 volts (6890 kPa @ 21°C [1000 psi @ 70°F]).
    • An AF ECU DTC is set.
    • Not all AF ECU DTCs cause a switch-over to gasoline.
    • A wide-open Mixture Control Valve (MCV) is detected.
    • Inspect the MCV control circuits for proper terminal contact and circuit continuity between the MCV and the GMS connectors
    • Engine crank time exceeds 8 seconds
    • The following conditions cause a start-up on gasoline followed by switch-over to natural gas during a vehicle decel condition:
       - An engine coolant temperature of less than (10°F) is detected at start-up
       - Every 100th engine start
  2. Check the fuel pressure sensor, the engine coolant temperature sensor, and the mixture control valve for proper operation. Check fuel pressure sensor accuracy by connecting the vehicle to a CNG filling station and, verifying tank pressure.
  3. Repair as necessary.

Was a problem found and repaired?

--

Go to Step 21

Go to Step 8

8

    Important: A higher secondary ignition voltage is required to operate the engine on Alternative Fuel than on gasoline. A vehicle that operates normally on gasoline does not indicate a fully functional ignition system.

  1. Inspect the ignition system for proper voltage output using the J 26792 spark tester.

    2. Important: Accurate CKP operation is required for Alternative Fuel operation. Check the CKP sensor on any vehicle that starts and immediately stalls.

  2. Inspect the Crankshaft Position Sensor (CKP) for proper sensor values and operation. Refer to the Enhanced Ignition System Diagnosis in the C/K Truck Service Manual.
  3. Remove and inspect the spark plugs for the following conditions:
    4. • Wet (fouled) plugs
    • Cracks
    • Excessive wear
    • Incorrect gap
    • Burned electrodes
    • Heavy deposits
  4. Inspect the spark plug wires for damage. Spray the secondary ignition with a water bottle, and check for crossfire or missing.
  5. Check the distributor cap for moisture or damage (if applicable).
  6. Check all the engine control modules (AF ECU, VCM/PCM or ECM, Ignition) grounds for being clean, tight, and properly located.
  7. Repair as necessary.

Was a repair necessary?

--

Go to Step 21

Go to Step 9

9

Perform the Fuel System Diagnosis procedure and return to this step.

Was a repair required in the AF Fuel System Diagnosis?

--

Go to Step 21

Go to Step 20

10

  1. Start and idle the engine.
  2. Using the scan tool, observe the FPS data parameter.
  3. Command the HPL open.

Does the FPS fuel pressure increase greater than the value specified when the HPL is opened?

8274 kPa (1200 psi)

Go to Step 15

Go to Step 11

11

Important: Before proceeding with FPS diagnosis ensure that the manual lock-down screw of the HPL manual shut-off fitting is open by rotating the allen screw counter-clockwise until fully seated.

  1. Verify the CNG fuel tank is at full pressure by connecting the vehicle to a CNG dispensing station and filling the CNG fuel tank.
  2. Turn ON the ignition, with the engine OFF.
  3. Compare the dispenser pressure gauge reading to the Fuel Pressure Sensor (FPS) scan tool parameter value.

Is the FPS accurately indicating fuel pressure?

--

Go to Step 12

Go to CNG Fuel Gauge Reading Inaccurate

12

  1. While monitoring the FPS data parameter, start and idle the engine ensuring that the engine is operating on CNG.
  2. Observe the FPS data parameter after a few minutes of engine operation.

Does the FPS data parameter ever drop below the pressure value specified?

8274 kPa (1200 psi)

Go to Step 19

Go to Step 21

13

  1. Using the scan tool, communicate with the gasoline control module (VCM/PCM or ECM) and locate the Natural Gas Operation data parameter.
  2. Turn OFF the ignition and wait 30 seconds.
  3. Disconnect the AF ECU connectors.
  4. Using a fused jumper connect the NGO Enable circuit connector terminal to a known good ground.
  5. Turn ON the ignition, but do NOT crank the engine.
  6. If the jumper fuse opens repair the short to voltage on the NGO Enable circuit.

Does the data parameter indicate that Natural Gas Operation is enabled?

--

Go to Step 16

Go to Step 14

14
  1. Test the NGO Enable circuit for an open circuit.
  2. Inspect for proper terminal contact at the gasoline control module connector.
  3. Inspect the VCM for the proper Bi-fuel calibration.
  4. Repair wiring or terminals, or program the VCM as necessary.

Was a repair necessary?

--

Go to Step 21

Go to Step 17

15

  1. Inspect the fuel system for a leak or for an LPL that does not hold pressure when the CNG fuel system is closed.
  2. Inspect the HPL for proper prime pulse operation.
  3. Repair as necessary.

Is the repair complete?

--

Go to Step 21

--

16
  1. Check terminal contact at the AF ECU connector.
  2. Repair terminal contact as necessary.

Was a repair necessary?

--

Go to Step 21

Go to Step 18

17

Important: The following control module replacement procedures must all be completed before attempting to operate the vehicle.

  1. Replace the gasoline control module (VCM/PCM or ECM). Refer to VCM Replacement/Programming .
  2. Program (flash) the replacement control module with the correct vehicle software and calibration.

    3. Important: In order to perform the variation learn procedure a switch-over to gasoline operation is necessary. Remove the CNG 20 amp fuse in order to perform the switch-over.

  3. Perform the Crankshaft Position System Variation Learn .

    4. All of the above procedures are located in the C/K Truck Service manual.

Is the replacement complete?

--

Go to Step 21

--

18
  1. Replace the AF ECU. Refer to Engine Control Unit Replacement .
  2. Program (flash) the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 21

--

19

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

Is the replacement complete?

--

Go to Step 21

--

20

Replace the GMS/MCV. Refer to Gas Mass Sensor/Mixture Control Valve Mounting Bracket Replacement .

Is the replacement complete?

--

Go to Step 21

--

21
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool, clear DTC Information from the AF ECU and the gasoline control module (VCM/PCM or ECM).
  4. Turn OFF the ignition and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the Fuel Indicator Lamp and the MIL.
  7. Observe vehicle performance and driveability.
  8. After operating the vehicle inspect BOTH Control Modules for stored DTCs.

Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs?

--

System OK

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