The alternative fuels supply system consists of the following components:

Fuel Tank

Two kinds of fuel tanks are available. Early 2005 model year vehicles have a Type 2 CNG fuel tank that is constructed of steel and is wrapped in carbon fiber set in epoxy. Late 2005 and later model year vehicles have a Type 4 CNG fuel tank that is constructed of compostite material and is wrapped in fiberglass and epoxy. A high pressure lock-off solenoid is contained within the fuel tank and is used to prevent fuel flow during non-cranking or running engine conditions. Fuel tanks have a protective cover. The cover is designed to protect the tank from damage due to shifting cargo and also for ultra-violet (UV) radiation protection. The cover is not designed to support a load. Federal government regulations require that the fuel tank and brackets be inspected every 3 years or 60 000 km (36,000 mi), whichever comes first. Fuel tank inspection results should be recorded in the CNG Fuel Tank Inspection Record area of the owners manual supplement. The service life of the CNG fuel tank can be 15 years or 20 years from the date of manufacture. All CNG fuel tanks regardless of inspection results must be replaced after their expiration date has passed. The CNG fuel tank expiration date and Tank Type identification is always listed on the tank label. Refer to Fuel Tank Inspection procedures.

Fuel Tank Pressure Relief Device (PRD)

The CNG fuel tank is equipped with a thermally activated PRD valve. The PRD is necessary in order to protect the CNG fuel tank from excessive pressure build-up in the event the tank is exposed to high temperatures. The PRD is sealed internally with a thermally sensitive metal valve. When the PRD is exposed to temperatures near 102°C (215°F), the metal valve will melt and allow the CNG fuel tank pressure to fully vent to the atmosphere. The PRD will not close once activated. On Type 2 fuel tanks the PRD is an internal, non-serviceable part of the high pressure lock-off (HPL) solenoid valve. On Type 4 fuel tanks the PRD is externally threaded into an open port of the HPL solenoid valve. The PRD port is not controlled by the HPL solenoid valve and is directly open to the internal tank pressure. The CNG fuel tank must be completely vented prior to removing the external PRD. The external PRD should be inspected regularly. Refer to Fuel Tank Inspection .

Fill Valve and Fill Line

The fill valve receives fuel from the fill nozzle at the CNG dispensing station. The fill valve seals to the dispensing nozzle with an O-ring. The O-ring must be inspected and replaced if missing or damaged prior to re-fueling. The fill valve contains an internal 380 micron filter element designed to trap contaminants. This filter must be periodically inspected and cleaned. Refer to Fill Valve Filter Replacement for filter inspection and cleaning procedures. The fill valve mounting location varies with vehicle model. Refer to the owner's manual supplement for fill valve location and fuel tank filling procedures. The fill line is constructed of stainless steel line. The fill line connects the fill valve to the fuel tank and is sealed to the fuel tank with an O-ring.

Fill Line Union Check Valve

The fill line is equipped with a check valve. In the event of fill valve leakage, the check valve is designed to minimize the amount of CNG fuel leakage. The check valve must be installed with the flow direction arrow pointed toward the high pressure lock-off solenoid.

High Pressure Lock-Off (HPL) Solenoid

The HPL is a normally closed, solenoid valve. The HPL solenoid and a low pressure lock-off (LPL) solenoid in the fuel line prevent fuel flow. The powertrain control module (PCM) commands ONLY the HPL open for 1 second at EVERY ignition ON in order to prime the CNG fuel system. The CNG prime pulse charges the fuel line which allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the system. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates that the engine is cranking or running on CNG.

Fuel Tank Pressure (FTP) Sensor

The FTP sensor is a pressure transducer. The fuel injector control module (FICM) supplies about 5 volts on the FTP reference circuit. The FICM also supplies a signal and ground circuit to the FTP. When the fuel tanks are full (high pressure) a high voltage signal will be monitored by the FICM. The FTP sensor is threaded into the fuel tank HPL solenoid. The FTP sensor voltage is sent to the PCM through a PWM circuit. The volume of CNG will vary with temperature and pressure. Accurate CNG fuel level cannot be determined by pressure only. In order to compensate for the different volume factors a fuel tank temperature (FTT) sensor is mounted within the HPL. The PCM performs a calculation on the fuel tank pressure vs. the in-tank temperature. The PCM will then display the temperature corrected CNG fuel level. The CNG FTP sensor should not be confused with the gasoline fuel tank pressure (FTP) sensor that is utilized for Evaporate (EVAP) emissions monitoring.

Fuel Tank Temperature (FTT) Sensor

The FTT sensor is a thermistor mounted inside the HPL and is not serviceable separately from the HPL. The FTT thermistor has high resistance when cold and low resistance when hot. The FICM supplies about 5 volts to the FTT sensor signal circuit. When the FTT sensor is cold its resistance is high and the signal voltage is high. As the FTT sensor warms and resistance drops, less signal voltage is monitored by the FICM. The FTT sensor voltage is sent to the PCM through a PWM circuit. The volume of CNG will vary with temperature and pressure. Accurate CNG fuel level cannot be determined by pressure only. In order to compensate for the different volume factors the fuel tank temperature (FTT) sensor is mounted within the HPL. The PCM performs a calculation on the fuel tank pressure vs. the in-tank temperature. The PCM will then display the temperature corrected fuel level.

High Pressure Line

The high pressure line is stainless steel tubing with matching high pressure O-ring face seal (ORFS) fittings. The individual tubes are pre-formed with fittings installed. All O-ring face seal fittings use nitrile rubber O-rings and must be replaced with the correct replacement part whenever a line is opened.

When installing pipe thread fittings DO NOT use Teflon® tape. Tape fragments could lodge in a regulator and allow fuel pressure to exceed specified levels. The use of pipe sealant with Teflon® is required.

Intermediate Pressure Fuel Hose

The intermediate pressure fuel hose assembly is a unique 2-piece stainless steel braided hose and is designed to isolate AF components from vibration.

Coalescing CNG Fuel Filter

The coalescing CNG fuel filter is located between the high pressure lock-off solenoid and the high pressure regulator. The coalescing filter housing contains a 6-micron filter element as well as a drain fitting. The coalescing filter is designed to trap contaminates and liquids that may damage the gaseous fuel injectors. This filter must be periodically inspected, drained and replaced.

The alternative fuels metering system consists of the following components:

High Pressure Regulator (HPR)

The HPR is supplied with fuel from the CNG fuel tank at up to 24 821 kPa (3,600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to between 758-1 448 kPa (110-210 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 1 896 kPa (275 psi) on the output stage of the HPR. The PRD is removable in order to allow the installation of a pressure gage for diagnostic purposes. The HPR has an internal, serviceable 40-micron filter. This filter must be serviced periodically. Refer to High Pressure Regulator Filter Replacement for replacement procedures.

Low Pressure Lock-Off (LPL) solenoid

The LPL is a normally closed, solenoid valve. The LPL is mounted in the Intermediate pressure line. The PCM controls LPL solenoid operation. The PCM energizes the LPL solenoid only when engine cranking or running is detected. Anytime the engine is stopped the PCM de-energizes the LPL solenoid in order to stop fuel flow. The PCM operates the LPL and the high pressure lock-off (HPL) solenoid at the same time except at ignition ON. During ignition ON only 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 758-1 448 kPa (110-210 psi). The outlet of the LPL supplies the intermediate pressure regulator (IPR) with fuel.

Intermediate Pressure Regulator (IPR)

Fuel enters the IPR at the intermediate stage pressure of 758-1 448 kPa (110-210 psi). The IPR reduces the fuel pressure down to about 276 kPa (40 psi). The fuel exits the IPR and flows to the CNG fuel rails. The IPR is equipped with a schrader valve for testing the inlet pressure. Refer to Fuel System Diagnosis for pressure checking procedures.

Gaseous Fuel Injectors

The gaseous fuel injectors are pulse-width modulated and sequentially fired. The injector voltage is supplied by an injector relay. The FICM controls the ground path.

Fuel Rail Pressure Sensor

The fuel rail pressure (FRP) sensor is a pressure transducer. The PCM supplies about 5 volts on the FRP sensor reference circuit. The PCM also supplies a signal and ground circuit to the FRP sensor. When fuel pressure is high the signal voltage is high. The FRP signal is used by the PCM for fuel trim adjustment.

Fuel Rail Temperature Sensor

The fuel rail temperature (FRT) sensor is a thermistor. The FRT thermistor has high resistance when cold and low resistance when hot. The FICM supplies about 5 volts to the FRT sensor signal circuit. When the FRT sensor is cold its resistance is high and the signal voltage is high. As the FRT sensor warms and resistance drops, less signal voltage is monitored by the FICM. The FRT sensor voltage is sent to the PCM through a PWM circuit. The FRT signal is used by the PCM for fuel trim adjustment.

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 and fill the gasoline level.
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 Indicator Message

The PCM indicates which fuel system is in operation by sending a serial data message to the instrument panel. The driver information message center is contained within the instrument panel. The PCM controls the operation of the alternative fuels related messages. The messages momentarily indicate which fuel system is in operation during engine start-up. A message indicating which fuel level is being shown will also be momentarily displayed when the vehicle operator presses the fuel gage select switch.

Fuel Gage Operation

The CNG fuel level is determined by the PCM. The PCM monitors the FTP and FTT sensors that are located on the CNG tank high pressure lock-off solenoids. The PCM performs a calculation of the CNG pressure and CNG temperature in order to determine an accurate CNG fuel level. The gasoline fuel level is determined by the PCM in a conventional manner using an in-tank variable resistor sensor. Under normal conditions the instrument panel fuel gage displays the level of fuel that the vehicle is operating on. A fuel gage selector switch allows the two different fuel system tank levels to be displayed by the one instrument panel fuel gage. The fuel gage selector switch is a momentary-closed type switch located to the right of the instrument panel. The fuel gage switch allows the vehicle operator to observe the fuel level of the non-operating fuel system. The PCM monitors the status of the switch. When the switch is pressed, the PCM changes the fuel level signal to display the non-operating system fuel level. Once requested, the non-operating fuel system level will be displayed for about 10 seconds. The PCM communicates fuel level to the instrument panel fuel gage through serial data.