Fuel Pump: Testing and Inspection

FUEL SYSTEM DIAGNOSIS

SYSTEM DESCRIPTION
The powertrain control module (PCM) controls the fuel system while the vehicle is operating on CNG. The PCM 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 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 prevent fuel flow. The PCM 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 tank pressure (FTP) sensor to monitor the amount of fuel pressure in the tank. The PCM 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 two different pressure stages in order to lower the fuel pressure from tank pressure down to an injector delivery pressure.

HIGH PRESSURE REGULATOR STAGE
When the PCM commands the HPL open fuel begins to flow through the high pressure regulator (HPR). The HPR reduces fuel pressure to between 758-1 448 kPa (110-210 psi). The outlet of the HPR is the intermediate pressure stage.

INTERMEDIATE PRESSURE REGULATOR STAGE
With a pressure between 758-1 448 kPa (110-210 psi) the fuel exiting the HPR flows through the intermediate fuel line to the LPL solenoid. The LPL is controlled by the PCM. Fuel flows out of the LPL and into the intermediate pressure regulator (IPR). The IPR reduces fuel pressure to between 246-317 kPa (43-46 psi). Fuel flows out of the IPR and into the fuel rails.

TEST DESCRIPTION

Step 1 - Step 4:

Step 5 - Step 7:

Step 8 - Step 11:

Step 12 - Step 15:

Step 16 - Step 19:

The numbers below refer to the step numbers on the diagnostic table.
2. This step verifies that the HPL solenoid and the FTP sensor are functioning. If the HPL solenoid is not functioning or if the manual tank valve is closed the dispensing station will charge only the fuel lines with pressure. Cranking or starting the engine will cause this pressure to decrease rapidly.
3. This step verifies the fuel rail pressure (FRP) sensor is displaying the correct fuel pressure.
4. This step tests for a leak in the fuel system between the LPL, and the fuel injectors. The fuel pressure should remain constant during the test.
5. This step tests the HPR output pressure. If the fuel pressure is within the specified values, then the HPR, fuel lines, and filters are OK.
6. This step tests the operation of the HPL.
8. This step tests the operation of the LPL.
15. This step verifies that a fuel filter is not the cause of the condition. If the fuel pressure is within the specified range after the fuel filter replacement, then a plugged filter was the cause of the condition.