Fuel System Unwanted Switch To Gasoline: Description

Alternate Fuel Engine Control Unit (AF ECU) and Powertrain Control Module (PCM) are connected by Natural Gas Operation (NGO) 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 ECU performs switch-over command by grounding NGO 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 Pressure Sensor (FPS) indicates CNG tank is empty.
• A wide-open Mixture Control Valve (MCV) is detected.
• System fault is detected by control module (AF ECU 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.