The common rail fuel system is comprised of 2 fuel pressure sections, a suction side between the fuel tank and the fuel supply pump, and a high pressure side between the fuel supply pump and the fuel injectors. Fuel is drawn from the fuel tank via a feed pump and then pumped into the fuel rail by 2 plungers, all of which are internal to the fuel supply pump. This high pressure is regulated by the engine control module (ECM) using the fuel rail pressure (FRP) regulator dependant upon values from the FRP sensor attached to the fuel rail. In case of fuel rail overpressure, a pressure limiter valve threaded into the fuel rail will open to release overpressure and return fuel back to the fuel tank.
The Fuel System Check diagnostic table directs the service technician to the appropriate fuel system diagnosis. The diagnostic table assumes the following conditions are met:
• | The batteries are completely charged. |
• | The engine cranking speed is normal. |
• | There is adequate fuel in the fuel tank. |
• | There is no air in the fuel line. |
• | The fuel system, from the fuel tanks to the fuel supply pump is under a slight vacuum with the engine running. As a result, air can enter the fuel system if these connections are not tight or if there is a crack in one of the fuel hoses. Air in the fuel system will cause fuel rail pressure fluctuations, especially at high engine speed and load. |
• | If the fuel tank is empty or near empty, air might be allowed to go into the fuel system. With air in the fuel system, smooth flow of the fuel into the supply pump is interrupted. Perform bleeding of the fuel system after refilling. |
• | The Actual Fuel Rail Pressure on the scan tool will only update with the engine running, or one time after the key is cycled when the ignition has been turned OFF for longer than 10 seconds. |
Step | Action | Values | Yes | No | ||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Are any DTCs set? | -- | Go to Step 2 | |||||||||||||||||
2 |
Important: Fuel may leak under the cylinder head cover from the inlet high pressure line. In such case, the engine oil level will rise. Inspect for fuel in the engine oil.
Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 3 | ||||||||||||||||
3 |
Important: The fuel system, from the fuel tanks to the fuel supply pump, is under a slight vacuum with the engine running. As a result, air can enter the fuel system if these connections are not tight. Air in the fuel system will cause fuel rail pressure fluctuations, especially at high engine speed and load. Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 4 | ||||||||||||||||
4 |
Is the difference between the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure less than the specified value? | 725 psi (5 MPa) | System OK | Go to Step 5 | ||||||||||||||||
5 |
Important: The fuel vacuum pump/pressure gage connector and the adapter hose must be cleaned before connecting to the fuel line. Foreign material internal to the tools line may damage the fuel supply pump. Important: Release the clamp or open the plug when the gage is likely to be more than 8 in Hg during the test. Can a vacuum of at least the specified amount be pulled on the fuel system? | 8 in Hg | Go to Step 8 | Go to Step 6 | ||||||||||||||||
6 |
Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 7 | ||||||||||||||||
7 |
Can a vacuum of at least the specified amount be pulled on the fuel system? | 8 in Hg | Go to Step 18 | Go to Step 17 | ||||||||||||||||
8 |
Does the J 44638 ever indicate a value larger than the specified value? | 5 in Hg | Go to Step 9 | Go to Step 10 | ||||||||||||||||
9 |
Did you find and correct the condition? | -- | Go to Step 18 | Go to Step 14 | ||||||||||||||||
10 |
Important: Perform this test 3 times and take the average of the fuel return amount coming from the cylinder head.
Is fuel leaking from the pressure limiter valve? | -- | Go to Step 15 | Go to Step 11 | ||||||||||||||||
11 | Is the average fuel return coming from the cylinder head less than the specified value? | 30 ml | Go to Step 12 | Go to Step 13 | ||||||||||||||||
12 |
Did you find wet fuel on any glow plug? | -- | Go to Step 16 | Go to Step 17 | ||||||||||||||||
13 |
Important: Replacement injector must be programmed. Did you complete the replacement? | 3 ml | Go to Step 18 | -- | ||||||||||||||||
14 | Replace the fuel filter cartridge. Refer to Fuel Filter Replacement . Did you complete the replacement? | -- | Go to Step 18 | -- | ||||||||||||||||
15 | Replace the pressure limiter valve. Refer to Fuel Injection Fuel Rail Assembly Replacement . Did you complete the replacement? | -- | Go to Step 18 | -- | ||||||||||||||||
16 |
Important: Replacement injector must be programmed. Replace the appropriate injectors that was leaking fuel found at step. Refer to Fuel Filter Replacement . Did you complete the replacement? | -- | Go to Step 18 | -- | ||||||||||||||||
17 |
Important: The fuel supply pump must be timed to the engine and adjustment value must be learned to the ECM. Replace the fuel supply pump. Refer to Fuel Injection Pump Replacement . Did you complete the replacement? | -- | Go to Step 18 | -- | ||||||||||||||||
18 |
Is the difference between the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure less than the specified value? | 725 psi (5 MPa) | System OK | Go to Step 1 |