Fuel System Unwanted Switch to Gasoline. GMC Truck GMC C Sierra

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C Sierra - 2WD (LPG) 🢒 Engine 🢒 Engine Controls - 6.0L 🢒 Fuel System Unwanted Switch to Gasoline

Circuit Description

The alternative fuels engine control module (ECM) and the powertrain control module (PCM) are connected by the alternative fuels operation (AFO) signal circuit. In order for the engine to start and operate on CNG, the PCM must be commanded to disable the gasoline fuel injectors, switch to a different ignition spark calibration and disable some of the PCM DTCs. The alternative fuels ECM performs the switch-over command by grounding the AFO signal circuit when CNG operation is desired.

The KL6 equipped 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 tank has less than 1379 kPa (200 psi).

•	The fuel rail pressure (FRP) sensor indicates low CNG fuel rail pressure.

•	A system fault is detected by the alternative fuels ECM.

•	Every 100th start the vehicle will start and run briefly on gasoline. After this, a switch-over to CNG will occur during the first decel condition.

•	The engine cranks for 8 seconds and fails to start on CNG.

Step

Action

Values

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)

Crank or idle the engine for the specified amount of time.
Observe the diagnostic trouble code (DTC) information from the alternative fuel ECM with a scan tool.

Does the scan tool display DTC P0201-208, P0602, P0605 or P1215 failed this ignition?

10 seconds

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

Observe the fuel indicator lamp (FIL) while the engine is running.

Is the FIL illuminated?

Go to Step 5

Go to Step 4

The condition is intermittent. Refer to Diagnostic Aids.

Is their a system operation concern?

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

System OK

With the scan tool, communicate with the alternative fuels ECM and observe the AFO Signal and AFO Signal Command data displays.
Observe the displays as you crank and start the engine.

Do the data displays indicate that the Alternative Fuels ECM is in control of engine fueling?

Go to Fuel Indicator Lamp Always On - Alternative Fuels in Instrument Panel, Gauges and Cluster

Go to Step 6

Do both displays indicate the same information?

Go to Step 7

Go to DTC P1215

Turn OFF the ignition.
Disconnect the harness connector of a CNG fuel injector.
Turn ON the ignition, with the engine OFF.
Probe the ignition feed circuit of the CNG fuel injector, with a test lamp connected to a known good ground.

Does the test lamp illuminate?

Go to Step 8

Go to Fuel Injector Circuit Diagnosis

Important: Before proceeding with fuel tank pressure (FTP) sensor diagnosis ensure that the manual lock-down screw of the HPL manual shut-off fitting is open by rotating the screw counter-clockwise until fully seated.

Verify the CNG fuel tank is at full pressure by connecting the vehicle to a CNG dispensing station and filling the CNG fuel tank.
Turn ON the ignition, with the engine OFF.
Observe the FTP scan tool data display.

Is the FTP sensor accurately indicating fuel pressure?

Go to Step 9

Go to Fuel Gauge Inaccurate or Inoperative - Alternative Fuels in Body and Accessories

Turn the ignition OFF and wait 15 seconds.
Turn ON the ignition, with the engine OFF.
With the scan tool, communicate with the alternative fuels ECM and observe the fuel rail pressure sensor (FRP) data display.
Observe the FRP data display while cranking the engine.

Does the FRP data display ever indicate fuel pressure greater than the value specified?

276 kPa (40 psi)

Go to Step 10

Go to Fuel System Diagnosis

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.
Remove and inspect the spark plugs for the following conditions:

•	Wet (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.
Inspect the following engine control module grounds for being clean, tight, and properly located:

•	alternative fuels ECM

•

PCM

Repair as necessary.

Did you find and correct the condition?

Go to Step 12

Go to Step 11

Replace the alternative fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Did you complete the replacement?

Go to Step 12

Connect any disconnected components, fuses, etc.
With the scan tool, clear DTC Information from the alternative fuels ECM and the powertrain control module (PCM).
Turn the ignition OFF and wait 30 seconds.
Start the engine and operate the vehicle.
Observe the Fuel Indicator Lamp (FIL) and the MIL.
Observe vehicle performance and driveability.
After operating the vehicle inspect the alternative fuels ECM and the PCM 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 Trouble Code (DTC) List or Symptoms - Engine Controls