Fuel System Diagnosis (CNG): Description

WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) controls the fuel system while the vehicle is operating on CNG. The PCM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in a cylindrical fuel tank. A full tank can contain fuel at 24,820 kPa (3600 psi). A High Pressure Lock-Off (HPL) solenoid in the tank and a Low Pressure Lock-Off (LPL) solenoid in the fuel line prevent fuel flow. The PCM commands ONLY the HPL open for one second at every ignition ON for a CNG prime pulse. The CNG prime pulse is performed to charge the fuel line and also allows the Fuel Tank Pressure (FTP) sensor to monitor the amount of fuel pressure in the tank. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates the engine is cranking or running on CNG.

This fuel delivery system utilizes two different pressure stages to lower the fuel pressure from tank pressure down to an injector delivery pressure.