The PCM controls fuel delivery and for KL6 vehicles, determines which fuel system operates the engine. The PCM monitors various engine and vehicle functions to provide the correct amount of CNG or gasoline fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining reduced emission levels. When operating on gasoline, the PCM will turn the fuel indicator lamp ON. The fuel indicator lamp (FIL) is located within the headlamp switch assembly near the fuel gage select switch. The FIL is also turned ON for a two-second bulb check whenever the ignition is turned ON. The fuel gage select switch allows the vehicle operator the ability to request fuel gage display of either fuel tank.

The PCM is connected to the FICM. The PCM is not capable of operating the high current CNG fuel injectors. The PCM injector pulse width signals are received by the FICM and duplicate pulse width signals are generated by the FICM in order to operate the CNG injectors. KL6 equipped vehicles utilize a FICM that contains internal relays. When operating on gasoline the internal relays allow the PCM injector pulse width to go directly to the gasoline injectors. Refer to Fuel Injector Control Module Description for more detailed description information.

The primary components of the CNG fuel SUPPLY system are as follows:

The CNG Fuel Tanks are constructed of high-strength steel covered with a two-stage epoxy coating finish. A high pressure lock-off solenoid is threaded into the end of each fuel tank and is used to prevent fuel flow during non-cranking or running engine conditions. The fuel tanks also have a protective cover. The cover is designed to protect the tank from road debris.

On passenger and cargo vans the fuel tanks are mounted in tandem, transverse behind the rear axle, under the vehicle. On cutaway vans the fuel tanks are mounted in tandem, parallel to the vehicle frame, behind the rear axle. Dedicated equipped vehicles (KL8) also utilize a third, mid-frame mounted tank that is located in place of the gasoline fuel tank.

Federal Government Regulations require that the fuel tank and brackets be inspected every three years or 60 000 km (36,000 miles), which-ever comes first. Fuel tank inspection results should be recorded in the CNG Fuel Tank Inspection Record area of the owners manual supplement (if applicable). The service life of the CNG fuel tank is 15 years from the date of manufacture. All CNG fuel tanks regardless of inspection results must be replaced after  15 years.

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. Before re-fueling this O-ring must be inspected and replaced if missing or damaged. The fill valve contains an internal filter element designed to trap contaminants. This filter must be periodically inspected and cleaned. The fill valve is mounted behind the fuel filler door. Refer to the owners manual supplement for fuel tank filling procedures.

The fill line is constructed of 3/8 in. flexible, steel-braided hose protected by a plastic cover. The fill line connects the fill valve to a line union check valve and is sealed at all connections with an O-ring.

In the event of fill valve leakage, the line union check valve is designed to minimize the amount of CNG fuel leakage.

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

The FTP sensor is a pressure transducer. The PCM supplies about 5 volts on the FTP reference circuit. The PCM 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 PCM. The FTP sensor is threaded into one of the rear fuel tank HPL solenoids.

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 monitors the FTT sensor and performs a calculation on the FTP voltage vs. the in-tank temperature. The PCM will then display the temperature corrected 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.

The FTT 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 PCM supplies about 5 volts to the FTT signal circuit. When FTT resistance is high (cold sensor) the FTT signal voltage is high. As the FTT warms and resistance drops, less signal voltage is monitored by the PCM.

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 monitors the FTT sensor and performs a calculation on the FTP voltage vs. the in-tank temperature. The PCM will then display the temperature corrected fuel level.

The CNG fuel system uses different fuel line types. The type used is dependent upon the pressure the fuel line is required to handle.

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 that are specific for gaseous fuel operation. These O-rings must be replaced with the correct replacement part whenever a line is opened.

Intermediate Pressure Fuel Hose

The intermediate pressure fuel hose assemblies are unique flexible lines for connection at fuel injector pressures. The individual lines connect using flared fittings.

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.

All hose, lines and fittings must be replaced ONLY with the correct replacement GMSPO part number.

The in-line fuel filter is located under the vehicle between the high pressure regulator and the low pressure lock-off (LPL) solenoid. The filter is serviced at regular intervals.

The HPL is a normally closed, solenoid valve. The solenoid provides a fuel shut-off to the fuel rails. The LPL solenoid is mounted to the right hand fuel rail. The LPL is only commanded ON when engine RPM indicates the engine is cranking or running on CNG.

The CNG fuel rails are constructed of aluminum and are mounted on either side of the intake manifold. Fuel enters the right fuel rail from the LPL solenoid that is mounted to the rail. A crossover fuel line connects the two fuel rails. The fuel rails are mounted in the same location as the gasoline fuel rails on KL8 equipped vehicles. The fuel rails are mounted slightly above the gasoline fuel rails on KL6 equipped vehicles. KL6 vehicles also utilize an injector tee in order to allow both fuels to inject into the same cylinder.

The primary components of the CNG fuel METERING system are as follows:

High Pressure Regulator (HPR)

The HPR is supplied with fuel from the CNG fuel tank at up to 24821 kPa (3600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to around 1241 kPa (180 psi). The pressure drop within the regulator causes fuel temperature to drop. The HPR is connected to the vehicle engine cooling system in order to prevent HPR freeze-up. The outlet of the HPR is the injector pressure stage. Fuel flows out of the HPR and through the inline filter and into the low pressure lock-off (LPL) solenoid. The HPR contains an over-pressure relief device (PRD) which will not allow pressure above 1896 kPa (275 psi) on the output stage of the HPR. The HPR has an internal, serviceable 40 micron filter. This filter must be serviced periodically.

Gaseous Fuel Injectors

The gaseous fuel injectors are pulse-width modulated and sequentially fired. The injector voltage is supplied by a fuse. The FICM controls the ground path. KL6 vehicles utilize an injector tee in order to allow both fuels to inject into the same cylinder. KL8 vehicles mount the injectors directly into the intake manifold port.

Fuel Rail Pressure Sensor

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

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 data is transmitted to the PCM through a PWM signal circuit. The FRT signal is used by the PCM in order to adjust fuel trim.

The primary fuel source of a Bi-fuel vehicle 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 Lamp (FIL)

The fuel indicator lamp (FIL) is contained within the headlamp switch assembly. The fuel indicator lamp indicates which fuel system the vehicle is operating on. The PCM controls the operation of the fuel indicator lamp. The fuel indicator lamp illuminates only when the engine is operating on gasoline. The fuel indicator lamp will also illuminate for several seconds during start-up as a bulb check.

The CNG fuel level is determined by the PCM. The PCM monitors the FTP and FTT sensors that are located on one of 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. On Bi-fuel (KL6) models the gasoline fuel level is determined by the PCM in a conventional manner using an in-tank variable resistor sensor.

Bi-Fuel (KL6) Fuel Gage Operation

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 within the headlamp switch assembly. 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 depressed 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.

The dedicated CNG fuel system operation is functionally the same as the bi-fuel CNG fuel system. The vehicle does not contain any gasoline fuel system components. A third CNG fuel tank is placed in the location of the frame mounted gasoline fuel tank. The vehicle does utilize a PCM that operates in conjunction with the FICM. The calibration programmed into the PCM has disabled the gasoline specific DTCs. The PCM still performs diagnostics on the vehicle sensors such as ECT, TP, IAT, etc.