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 (3600 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.
When the PCM commands the HPL open fuel begins to flow through the high pressure regulator (HPR). The HPR reduces fuel pressure to between 758-1448 kPa (110-210 psi). The outlet of the HPR is the intermediate pressure stage.
With a pressure between 758-1448 kPa (110-210 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 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.
The numbers below refer to the step numbers on the diagnostic table.
This step verifies that the HPL solenoid and the FTP sensor are functioning. A fuel tank pressure value that decreases while the engine is operating indicates the HPL is not opening or the manual tank valve is closed.
This step verifies the FRP sensor is displaying the correct fuel pressure.
This step tests for a leak in the fuel system between the low pressure lock-off (LPL), and the fuel injectors. The fuel pressure should remain constant during the test.
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.
This step tests the operation of the HPL.
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.
This step determines if the FRP sensor or actual fuel rail pressure is the cause of the condition.
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.
Step | Action | Value(s) | Yes | No | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||||||
1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) | ||||||||||
Does the FTP sensor display a steady pressure at or above the specified value? | 8 274 kPa (1,200 psi) | Go to Step 3 | Go to Step 6 | |||||||||||
Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Is the FRP sensor value within the specified range? | 276-345 kPa (40-50 psi) | Go to Step 4 | Go to Step 8 | |||||||||||
Does the fuel rail pressure drop more than the specified value? | 21 kPa (3 psi) | Go to Step 12 | Go to Step 5 | |||||||||||
Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Important: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.
Are all of the fuel pressure values within the specified range? | 758-1 448 kPa (110-210 psi) | Go to Intermittent Conditions | Go to Step 14 | |||||||||||
Can operation of the HPL be heard? | -- | Go to Step 7 | Go to Step 20 | |||||||||||
7 |
Did you have to open the manual tank valve? | -- | Go to Step 2 | Go to Step 16 | ||||||||||
Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Important: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive. Is the pressure value within the specified range? | 758-1 448 kPa (110-210 psi) | Go to Step 10 | Go to Step 9 | |||||||||||
9 | Is the fuel pressure greater than the specified value? | 1 448 kPa (210 psi) | Go to Step 18 | Go to Step 14 | ||||||||||
10 |
Can operation of the LPL be heard? | -- | Go to Step 11 | Go to Step 19 | ||||||||||
Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Is the FRP sensor pressure within the specified value range? | 276-345 kPa (40-50 psi) | Go to Step 21 | Go to Step 17 | |||||||||||
12 |
Did you find and correct the condition? | -- | Go to Step 21 | Go to Step 13 | ||||||||||
13 |
Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
14 |
Did you find and correct the condition? | -- | Go to Step 21 | Go to Step 15 | ||||||||||
Important: If the engine will not start on CNG use the pressure values observed during cranking and proceed directly to the Step question. Important: Rhythmic pressure fluctuation of 10-20 psi is normal. A defective HPR is not indicated unless the fluctuations become excessive.
Are all fuel pressure values within the specified range? | 758-1 448 kPa (110-210 psi) | Go to Step 21 | Go to Step 18 | |||||||||||
16 | Replace the FTP sensor. Refer to Fuel Tank Pressure Sensor Replacement . Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
17 | Replace the IPR. Refer to Intermediate Pressure Regulator Replacement . Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
18 | Replace the HPR. Refer to Compressed Natural Gas High Pressure Regulator Replacement . Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
19 | Replace the LPL solenoid. Refer to Low Pressure Lock-Off Solenoid Replacement . Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
20 | Replace the HPL solenoid. Refer to High Pressure Lock-Off Solenoid Replacement . Did you complete the replacement? | -- | Go to Step 21 | -- | ||||||||||
21 |
Does the vehicle operate on CNG with normal driveability and no MIL illumination? | -- | System OK | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |