High Pressure Lock-Off (HPL) Solenoids

The HPL is a normally closed, solenoid valve. Both HPL solenoids, and a Low Pressure Lock-off (LPL) solenoid in the fuel line, prevent fuel flow. The AF ECM commands ONLY the HPLs open for 1 second at EVERY ignition ON for a CNG prime pulse. The CNG prime pulse is performed in order to charge the fuel line and also allows the Fuel Tank Pressure (FTP) sensor to monitor the amount of fuel pressure in the line. The AF ECM commands BOTH the HPLs and the LPL open when engine RPM indicates the engine is cranking or running on CNG.