Fuel System Unwanted Switch To Gasoline: Description

WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2002 Saturn SL2, 2002 Saturn SL1, 2002 Saturn SL, 2002 Saturn SC2, and 2002 Saturn SC1. However, it is still accessible from the selected car via links, so may be relevant.

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.