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

60 Day Cycle

It is recommended that every 60 days the vehicle be cycled between CNG and gasoline. This ensures the gasoline fuel system stays in good working condition and also accommodates for seasonal changes in gasoline formulation. Perform the following steps every 60 days.

1. With the vehicle operating on CNG check the gasoline level (fill if needed).
2. Operate the vehicle until the CNG fuel tank is empty and the vehicle switches over to gasoline operation.
3. Operate the vehicle until the gasoline tank is near empty.
4. Refill both fuel tanks.
5. The vehicle will switch-over to CNG operation.
6. Repeat procedure in 60 days.

Fuel Gauge Operation

This vehicle uses a fuel gauge select switch in order to allow the two fuel tank level readings (CNG and gasoline) to be displayed by one fuel gauge. With the ignition ON the fuel gauge will display the level for the fuel system currently in operation. When the selector switch is depressed the fuel gauge will display the level for the fuel system that is NOT in use. The fuel level for the non-operating fuel system will be displayed for about 10 seconds. Passenger vehicles have a filtered fuel level display, depress the fuel gauge select switch 30 seconds after the vehicle speed sensor indicates the vehicle is at rest in order to avoid inaccurate gauge displays. When operating on gasoline a fuel indicator lamp (FIL) located within the selector switch will illuminate. The fuel indicator lamp (FIL) will also illuminate for several seconds during engine start-up as a bulb check.

The alternative fuel engine control unit (AF ECU) controls the fuel system while the vehicle is operating on CNG. The AF ECU 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 3600 psi (24,820 kPa). A high pressure lock-off (HPL) solenoid in the tank and a low pressure lock-off (LPL) solenoid in the fuel line prevents fuel flow until the AF ECU monitors that the engine is cranking or running. This fuel delivery system utilizes three different pressure stages in order to lower the fuel pressure from tank pressure down to a delivery pressure which is slightly above atmospheric pressure.

High Pressure (tank) Stage

When the AF ECU commands the HPL open fuel begins to flow (at up to 3600 psi) through the high pressure steel lines up to the high pressure regulator (HPR). The HPR reduces fuel pressure to between 130-220 psi. The outlet of the HPR is the Intermediate pressure stage. The pressure drop within the regulator also causes fuel temperature to drop. In order to prevent HPR freeze-up, the HPR is connected to the vehicle engine cooling system.

Intermediate Pressure Stage

The Intermediate Pressure Stage begins at the outlet of the high pressure regulator (HPR). With a pressure between 130-220 psi, the fuel exiting the HPR flows through the intermediate fuel line to the low pressure lock-off (LPL) solenoid. The LPL is controlled by the AF ECU. Fuel flows out of the LPL through another portion of Intermediate fuel line and into the low pressure regulator (LPR). The inlet of the LPR is the Low Pressure Stage.

Low Pressure Stage

The low pressure regulator (LPR) is a two stage regulator. The fuel enters the LPR at the intermediate stage pressure of 130-220 psi. The first stage of LPR operation reduces the fuel pressure down to between 4 and 6 psi. The fuel pressure is then again reduced in the second stage of the LPR down to between 4.5-7 inches of water column. The fuel exits the LPR and flows through the low pressure hose and into the gas mass sensor / mixture control valve (GMS / MCV). The AF ECU monitors the gas mass sensor information and adjusts the amount of fuel entering the engine by varying fuel flow through the mixture control valve. The fuel exits the GMS/MCV and flows through another portion of low pressure hose and into the gas distribution adaptor (GDA). The fuel mixes with inlet air and is drawn through the intake manifold to be burned by the engine.

The Alternative Fuels METERING system contains the following components:

Refer to Fuel Supply Component Description for additional information regarding other alternative fuels components.

High Pressure Regulator (HPR)

The HPR is supplied with fuel from the CNG fuel tank (at up to 3600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to between 130-220 psi. The outlet of the HPR is the Intermediate pressure stage. Fuel flows out of the HPR and into the low pressure lock-off (LPL) solenoid. The pressure drop within the regulator causes fuel temperature to drop. In order to prevent HPR freeze-up, the HPR is connected to the vehicle engine cooling system. The HPR contains an over-pressure relief device (PRD) which will not allow pressure above 400 psi on the output stage of the HPR. The PRD is removable in order to allow the installation of a pressure gauge for diagnostic purposes. The HPR has an internal, serviceable 40 micron filter. This filter must be serviced periodically. Refer to Maintenance and Lubrication in this supplement for service intervals.

Low Pressure Lock-Off (LPL) solenoid

The LPL is a normally closed, solenoid valve. The LPL is mounted in the Intermediate pressure line. The AF ECU controls LPL solenoid operation. The AF ECU energizes the LPL solenoid only when engine cranking or running is detected. Anytime the engine is stopped the AF ECU de-energizes the LPL solenoid which stops fuel flow. The AF ECU operates the LPL and the high pressure lock-off (HPL) solenoid at the same time except at ignition ON. During ignition ON the HPL is opened while the LPL remains closed in order to charge the fuel system with CNG. The LPL is supplied fuel from the HPR with fuel pressure between 130-220 psi. The outlet of the LPL supplies the low pressure regulator (LPR) with fuel.

Low Pressure Regulator (LPR)

The low pressure regulator (LPR) is a two stage regulator. The fuel enters the LPR at the intermediate stage pressure of 130 to 220 psi. The first (primary) stage of LPR operation reduces the fuel pressure down to between 4-6 psi. The fuel pressure is then again reduced in the second (secondary) stage of the LPR down to between 4.5-7 inches of water column or around 1.5 kPa (0.21 psi). The fuel exits the LPR and flows through the low pressure hose and into the gas mass sensor/mixture control valve (GMS/MCV). The LPR contains two 3.175 mm (1/8 in) NPT test ports with plugs. The plugs are removable in order to allow the installation of a pressure gauge for diagnostic purposes. One port tests the first (primary) stage and the other port tests the second (secondary) stage. Refer to AF Fuel System Diagnosis for pressure checking procedures.

Gas Mass Sensor/Mixture Control Valve (GMS/MCV)

The GMS/MCV combines a hot wire type anemometer (GMS) and a magnetic rotary valve (MCV) into one non-serviceable unit. The GMS/MCV is mounted in the low pressure hose between the low pressure regulator (LPR) and the gas distribution adaptor (GDA). The GMS measures the mass fuel flow and outputs a frequency signal to the alternative fuels engine control unit (AF ECU). The AF ECU process this signal to determine the amount of fuel entering the engine. The AF ECU compares the GMS information as well as the information of other engine sensors and determines an optimum fuel flow rate to maintain a stoichiometric air/fuel ratio (16.5 to 1). In order to change the fuel mixture (rich or lean) the AF ECU modulates the MCV. The MCV will then allow less or more fuel into the engine.

Gas Distribution Adaptor (GDA)

The gas distribution adaptor (GDA) replaces the gasoline only air inlet duct. The GDA houses a gas ring which has distribution slots. The slots allow the CNG fuel to mix evenly into the intake air stream.