The fuel flows through a pre-filter screen in the tank and to the engine through the fuel supply lines. There is no lift pump in the fuel tanks or on the frame. The fuel passes through the base plate of the fuel injector control module (FICM) to cool the module. From the FICM the fuel flows to the fuel filter assembly, which combines a water separator, a prime pump and a filter element. Within the assembly, there is also a fuel heater that operates at fuel temperatures of 6° C (42° F) and below. An integrated hand pump is used to prime the fuel system after changing the fuel filter or servicing the fuel system. The fuel injection pump at the front of the engine valley includes a fuel supply pump and a high-pressure pump. Fuel is drawn to the supply pump from the primary fuel tank by the supply pump and delivered to the high-pressure pump.

The much larger section of the pump assembly is the high-pressure fuel injection pump. The pump is engine-driven by the camshaft gear. From the high-pressure pump, the fuel flows through the junction block. The junction block routes pressurized fuel to both common fuel rails. Each common fuel rail supplies one bank of four fuel injectors.

The Fuel Return System routes fuel from the fuel injectors, the junction block, and the fuel injection pump. The return fuel travels to the fuel cooler and then to the fuel tank. This fuel is used to cool and lubricate the injection pump and the injectors.

From the Engine to the Tank

1. Disconnect the return line at the engine, metal collar.
2. Remove the fuel filler cap.
3. Apply 1 psi air at the fuel return line that leads back to the tank.
4. Listen at filler tube for air.
5. If air is present, return line is okay.

If air is not present, disconnect the return line at the sender assembly.

6. Retest using 1 psi at the fuel return line back to the tank.

•	If air is present, the return line is okay. Check and replace the sender assembly.

•	If air is not present, repair the restriction in the return line.

From the Engine to the Injector Spill Lines

1. Disconnect the fuel return line (1) at the engine. Refer to Metal Collar Quick Connect Fitting Service .
2. Remove the air duct from the air cleaner assembly to the turbo inlet.
3. Remove the air intake pipe. Refer to Air Intake Pipe Replacement .



4. Remove the fuel return rubber hose of the fuel injection pump from the junction block (2). Cap the fitting on the block with a 3/8 inch rubber cap (1) to prevent fuel leakage.
5. Install the J 23738-A Vacuum Pump at the fuel return pipe.
6. Apply 15 inches Hg of vacuum to the pipe.

•	If the vacuum holds, go to step 13.

•	If the vacuum does not hold, go to step 7.

7. Remove the generator. Refer to Generator Replacement in Engine Electrical.



8. Remove the fuel return line (1) connecting the right front and left rear cylinder heads.



9. Install the J 23738-A onto the right front cylinder head return port (1).
10. Apply 15 inches Hg of vacuum to the port.

•	If the vacuum holds, go to step 11.

•

If the vacuum does not hold, the leak is in the spill lines under the valve cover. Install the fuel return line between the cylinder heads, remove the lower right valve cover, and go to step 15 of Fuel Leaks (inside of Engine). Refer to Valve Rocker Arm Cover Replacement - Lower Right Side in Engine Mechanical and Fuel Leak Diagnosis .

11. Install the J 23738-A onto the left rear cylinder head return port.



12. Remove the cap (1) from the junction block (2).
13. Apply 15 inches Hg of vacuum to the port.

•	If the vacuum holds, replace the leaking fuel return line between the cylinder heads.

•

If the vacuum does not hold, the leak is in the spill lines under the valve cover. Install the fuel return line between the cylinder heads, remove the lower right valve cover, and go to step 15 of Fuel Leaks (inside of Engine). Refer to Valve Rocker Arm Cover Replacement - Lower Left Side in Engine Mechanical and Fuel Leak Diagnosis .

14. Reinstall the fuel injection pump fuel return hose on the junction block and clamp the hose.
15. Install the air duct on the air cleaner assembly to the turbo inlet.
16. Install the air intake pipe. Refer to Air Intake Pipe Replacement .
17. Reinstall the fuel return line to the engine. Refer to Metal Collar Quick Connect Fitting Service .
18. Install the generator. Refer to Generator Replacement in Engine Electrical.
19. Start and idle the engine, and check for leaks.