Fuel System Diagnosis - BI-Fuel/Alternative Fuels

Fuel Pressure Gauge Connection:

SYSTEM DESCRIPTION
The Alternative Fuel Engine Control Unit (AF ECU) controls the fuel system while the vehicle is operating on alternative fuel. 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 prevent fuel flow. The AF ECU commands ONLY the HPL 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 Pressure Sensor (FPS) to monitor the amount of fuel pressure in the tank. The AF ECU 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 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 through the High Pressure Regulator (HPR). The HPR reduces fuel pressure to between 115-220 psi (793-1517 kPa). The outlet of the HPR is the intermediate pressure stage.

INTERMEDIATE PRESSURE STAGE
With a pressure between 115-220 psi (793-1517 kpa) 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 is a two stage regulator. The fuel enters the LPR at the intermediate stage pressure of 115-220 psi (793-1517 kpa). The Primary stage of LPR operation reduces the fuel pressure down to between 4-6 psi (28-41 kpa). The fuel pressure is then again reduced in the Secondary stage of the LPR down to between 2-5 inches (5.1-12.7 cm) 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).

TEST DESCRIPTION

Steps 1-2:

Steps 3-4:

Step 5:

Steps 6-9:

Steps 10-13:

Steps 14-20:

The numbers below refer to the step numbers on the diagnostic table.
3. This step tests the fuel pressure output of the High Pressure Regulator (HPR).
5. This step tests the fuel pressure output of the Primary Stage of the Low Pressure Regulator (LPR).
6. CNG fuel may contain small amounts of till station compressor oil. As the CNG fuel pressure drops in the LPR secondary chamber this oil separates from the CNG fuel and collects in the secondary chamber. Allow the oil to drain as this condition is normal and no repair is required.

This step tests the fuel pressure output of the Secondary Stage of the Low Pressure Regulator (LPR).
8. This step tests for the sound of a functioning LPL. A circuit failure to solenoid should set a DTC.
9. This step tests for the sound of a functioning HPL. A circuit failure to solenoid should set a DTC.
10. This step tests for a restricted high pressure regulator (HPR) filter. If a restricted HPR filter is found also inspect the fill valve filter for contamination, damage or restriction.
11. This step inspects for a closed manual lock-down screw.
12. This step inspects for damage to the fuel line between the HPL and the LPR.
19. This step tests for a leaking LPL. A leaking LPL would allow CNG fuel to enter the engine after the ignition has been turned OFF. The leaking LPL could cause a hard or no-start condition during the next ignition cycle. A leaking LPL would cause the gauge to indicate fuel pressure for up to several minutes after the ignition has been turned OFF.
20. This step tests for proper operation of the system after repairs are complete.