Fuel System Unwanted Switch To Gasoline: Description

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Alternate Fuel Engine Control Module (AF ECM) and Powertrain Control Module (PCM) are connected by the Alternative Fuels Operation (AFO) Enable circuit. For engine to switch-over to CNG operation, PCM must be commanded to disable gasoline fuel injectors, switch to a different ignition spark calibration and disable some PCM DTCs. AF ECM performs switch-over command by grounding AFO enable circuit when CNG operation is desired.

This vehicles primary fuel source is compressed natural gas (CNG). Vehicle will only switch-over to gasoline operation if one of the following conditions is present:

Fuel Tank Pressure (FTP) sensor indicates CNG tank is empty.
A wide-open Mixture Control Valve (MCV) is detected.
System fault is detected by control module (AF ECM or PCM).
Manual shut-off valve is closed.
Engine crank time exceeds 8 seconds.
On every 100th start and also when coolant temperature is less than 10°F (-12°C), vehicle will start and run briefly on gasoline. A switch-over to CNG will occur only during a decel condition.