Fuel System Diagnosis

SYSTEM DESCRIPTION
The alternative fuel engine control module (Alternative Fuels ECM) controls the fuel system while the vehicle is operating on alternative fuel. The Alternative Fuels ECM receives signals from various engine sensors and determines the correct amount of fuel required by the engine. The fuel is stored in cylindrical fuel tanks. A full tank can contain fuel at 24,820 kPa (3,600 psi). A High Pressure Lock-off (HPL) solenoid in the tank and a Low Pressure Lock-off (LPL) solenoid in the fuel line both allow and prevent fuel flow. The Alternative Fuels ECM commands ONLY the HPLs open for about 2 seconds at every ignition cycle 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 tanks. The Alternative Fuels ECM commands BOTH the HPLs 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 below atmospheric pressure.

High Pressure (tank) Stage
When the Alternative Fuels ECM commands the HPL open fuel begins to flow (at up to 3,600 psi) through the High Pressure Regulator (HPR). The HPR reduces fuel pressure to between 130 and 220 psi. The outlet of the HPR is the intermediate pressure stage.

Intermediate Pressure Stage
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 Alternative Fuels ECM. Fuel flows out of the LPL 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 2-stage regulator. The fuel enters the LPR at the intermediate stage pressure of 130-220 psi. The primary stage of LPR operation reduces the fuel pressure down to about 5 psi. The fuel pressure is then again reduced in the secondary stage of the LPR down to 0.5 inches of water column. The fuel exits the LPR and flows through the low pressure hose and into the fuel mixer.

Fuel Control Solenoid (FCS)
The FCS is an electronically controlled solenoid used to trim fuel mixtures by metering air valve vacuum to control the pressure on top of the LPR secondary diaphragm. This changes the LPR output pressure, and the amount of fuel flowing out of the LPR. The Alternative Fuels ECM controls the FCS with an electrical duty cycle which is converted into the regulated air valve vacuum signal communicated to the top of the LPR. As the FCS duty cycle increases, more vacuum on top of the LPR secondary diaphragm reduces the fuel flow out of the LPR which leans the air fuel mixture. As the FCS duty cycle decreases, less vacuum on top of the LPR secondary diaphragm increases the fuel flow out of the LPR, enriching the air fuel mixture. The fuel control authority of the FCS increases with engine air flow.

Idle Control Solenoid (ICS)
The ICS is an electronically controlled solenoid used to meter supplemental fuel from the LPR to the mixer to increase fuel control authority during idle and low speed operation. The Alternative Fuels ECM controls the ICS with an electrical duty cycle which is converted into the regulated fuel flow from the LPR to the mixer. As the ICS duty cycle increases, more supplemental fuel flows out of the LPR into the mixer, enriching the air fuel mixture. As the ICS duty cycle decreases, less supplemental fuel flows out of the LPR into the mixer which leans the air fuel mixture. The fuel control authority of the ICS decreases with engine air flow.

DIAGNOSTIC AIDS
The Fuel Control Solenoid (FCS) and Idle Control Solenoid (ICS) connectors are inter-changeable. Ensure that the RED connector is connected to the ICS. The ICS is the most rearward solenoid.

The Low Pressure Regulator (LPR) reference hose connects the LPR to the air cleaner outlet duct. This easily dislodged hose can cause driveability concerns if disconnected or restricted.

TEST DESCRIPTION

Steps 1-3:

Steps 4-9:

Steps 10-13:

Steps 14-17:

Steps 18-25:

Steps 26-28:

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).
7. CNG fuel may contain small amounts of fill 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. 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. The engine must be flowing the desired amount of airflow in order to accurately observe the secondary fuel pressure.
13. This step performs a road-load test of the high pressure regulators ability to supply fuel at higher flow rates.
14. This step inspects 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.
15. This step tests the ability of the Alternative Fuels ECM to energize the High Pressure Lock-off (HPL) solenoid.
16. This step inspects for closed manual lock-down screws.
17. This step inspects for the sound of a functioning HPL.
18. This step inspects for damage to the fuel line between the HPL and the HPR.
19. This step tests the ability of the Alternative Fuels ECM to energize the Low Pressure Lock-off (LPL) solenoid.
20. This step inspects for the sound of a functioning LPL.
21. This step checks for damage to the fuel line between the HPR and the LPR.
28. This step inspects for proper operation of the system after repairs are complete.