Fuel System Unwanted Switch to Gasoline. Chevrolet Express (CNG) 2001

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel System Unwanted Switch to Gasoline

Circuit Description

The Alternative Fuels Engine Control Module (AF ECM) and the gasoline control module (VCM/PCM or ECM) are connected by the Alternative Fuels Operation (AFO) signal 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 Alternative Fuels ECM performs the switch-over command by grounding the AFO Signal 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 tank pressure (FTP) sensor indicates the CNG tanks are empty.

•	A system fault is detected by the Alternative Fuels ECM.

•	The engine crank time exceeds 8 seconds.

•	Every 100th start the vehicle will start and run on gasoline. A switch-over to CNG will occur during the first 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:

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

At the same moment that the HPLs are opened the Alternative Fuels ECM 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 Alternative Fuels ECM.

Detecting the low fuel pressure in the system, the Alternative Fuels ECM performs a switch-over to gasoline.

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

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 Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

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

The Diagnostic System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.

This step tests if the Alternative Fuels ECM has commanded the gasoline control module, using the AFO Signal circuit, to start on CNG.

The AFO Signal Command data parameter is the commanded state of the AFO Signal circuit from the Alternative Fuels ECM. The AFO Signal data parameter is the monitored state of the AFO Signal circuit from the GASOLINE control module (VCM/PCM or ECM).

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

This step inspects for an AFO Signal circuit failure or for a PCM utilizing standard gasoline software and calibration.

This step inspects if the CNG fuel tank pressure (FTP) sensor is monitoring adequate fuel pressure for the engine to start on CNG. Once fuel pressure drops below about 200 psi (70°F) the Alternative Fuels ECM will not attempt to start on CNG until fuel pressure is greater than 1,000 psi (70°F).

This step tests 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.

This step tests for proper fuel tank pressure sensor operation.

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check - Alternative Fuels - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

With a scan tool, communicate with the Alternative Fuels ECM and observe the AFO Signal and AFO Signal Command data parameters.
Observe the data parameters as you crank and start the engine.

Do both data parameters indicate that the AF ECM is in command of engine fueling?

Go to Step 3

Go to Step 5

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

Is the FIL illuminated?

Go to Fuel Indicator Lamp Always On - Alternative Fuels in Instrument Panels, Gages, and Console

Go to Step 4

The fault is not present. Refer to Diagnostic Aids.

Is their a system operation concern?

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

System OK

Do the AFO Signal and AFO Signal Command data parameters display different information?

Go to Step 13

Go to Step 6

Observe the alternative fuels Fuel Tank Pressure (FTP) sensor data parameter.

Does the Fuel Tank Pressure sensor indicate CNG fuel pressure of more than the value specified?

8274 kPa (1,200 psi)

Go to Step 7

Go to Step 10

Start and idle the engine.
Command the low pressure lock-off (LPL) solenoid ON.
Listen for LPL operation.

The sound of CNG fuel passing through the LPL should be audible.

The engine may also stall if enough CNG fuel is present.

Does the low pressure lock-off solenoid operate as commanded?

Go to Step 9

Go to Step 8

Remove the engine cover. Refer to Instrument Panel Extension Replacement in Body and Accessories in the G Van Service Manual.
Start and idle the engine.
Grasp the low pressure lock-off (LPL) solenoid so that you can feel for LPL operation.
Command the low pressure lock-off solenoid ON.

Can you feel the low pressure lock-off solenoid operate?

Go to Fuel System Diagnosis

Go to Step 16

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.

Inspect the ignition system for proper voltage output using the J 26792 Spark Tester.

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

Inspect the Crankshaft Position Sensor (CKP) for proper sensor values and operation. Refer to the Electronic Ignition (EI) System Diagnosis in Engine Controls in the G Van Service Manual.
Remove and inspect the spark plugs for the following conditions:

•	Wet or fouled plugs

•

Cracks

•	Excessive wear

•	Incorrect gap

•	Burned electrodes

•	Heavy deposits

Inspect the spark plug wires for damage. Spray the secondary ignition with a water bottle, and check for crossfire or missing.
Check the distributor cap for moisture or damage, if applicable.
Check all the engine control modules (AF ECM, VCM/PCM or ECM, Ignition) grounds for being clean, tight, and properly located.
Repair as necessary.

Was a repair necessary?

Go to Step 19

Go to Fuel System Diagnosis

Ensure that both of the manual shut-off fittings are open by turning them fully counterclockwise.

The manual shut-off fittings are allen screws located on the end of each high pressure lock-off solenoids. An access hole is provided in the fuel tank mounting frame.

Did you have to open the manual shut-off fittings?

Go to Step 12

Go to Step 11

Turn the ignition ON while leaving the engine OFF.
Command the high pressure lock-off (HPL) relay ON and OFF using the scan tool.
Listen for audible HPL solenoid.

Do the high pressure lock-off solenoids operate as commanded?

Go to Step 12

Go to Fuel Lock-Off Relay Diagnosis

Ensure that the fuel tank pressure (FTP) is correctly indicating CNG fuel level by filling the CNG fuel tanks.
Cycle the ignition ON and OFF several times.
Turn the ignition ON while leaving the engine OFF.
Observe the fuel tank pressure (FTP) sensor data parameter after the CNG tanks are full.

Does the fuel tank pressure (FTP) sensor data parameter indicate that the CNG tanks are full?

3,600 psi@70°F

Go to Step 2

Go to Fuel Gauge Inaccurate or Inoperative - Alternative Fuels in Instrument Panel, Gages, and Console

Does the AFO Signal indicate that the PCM is in command?

Go to Step 14

Go to Step 15

Inspect the gasoline control module (PCM, VCM or ECM) for the proper Bi-fuel calibration.
Test the AFO Signal circuit for an open circuit.
Inspect for proper terminal contact at the gasoline control module connector.
Test the AFO Signal circuit for a short to battery positive.
Repair wiring or terminals, or program the gasoline control module as necessary.

Was a repair necessary?

Go to Step 19

Go to Step 17

Test the AFO Signal circuit for a short to ground.
Inspect for proper terminal contact at the Alternative Fuels ECM.
Repair wiring or terminals as necessary.

Was a repair necessary?

Go to Step 19

Go to Step 18

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

Is the replacement complete?

Go to Step 19

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

Replace the gasoline control module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement .
Program the replacement control module with the correct BI-FUEL vehicle software and calibration.

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.

Perform the Crankshaft Position System Variation Learn .

All of the above procedures are located in the G Van Service Manual.

Is the replacement complete?

Go to Step 19

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
Program the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

Go to Step 19

Remove all test equipment except the scan tool.
Connect any disconnected components, fuses, etc.
Using a scan tool, clear DTC Information from the Alternative Fuels ECM and the gasoline control module (VCM/PCM or ECM).
Turn OFF the ignition and wait 30 seconds.
Start the engine and operate the vehicle.
Observe the Fuel Indicator Lamp and the MIL.
Observe vehicle performance and driveability.
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 Diagnostic System Check - Alternative Fuels (Engine Controls)