Fuel is drawn by the fuel injection pump through a pre-filter screen in the tank. Fuel flows to the diesel fuel conditioning module (DFCM) , which combines a water separator, an electric fuel prime pump, fuel heater element, and a filter element. The fuel is then delivered to a second screw on fuel filter and the high-pressure pump. An integrated electric fuel prime pump is used to prime the fuel system after changing the fuel filter or servicing the fuel system. The mechanical fuel injection pump at the front of the engine valley includes the fuel supply pump and the high-pressure pump.

The much larger section of the fuel injection pump assembly is the high-pressure fuel injection pump. The pump is engine-driven by the camshaft gear. From the high-pressure pump, the pressurized fuel flows to the left fuel rail. A balance pipe from the center of the left fuel rail then feeds the right fuel rail. Each fuel rail supplies one bank of 4 fuel injectors. The fuel rail pressure (FRP) sensor is mounted in the right fuel rail.

Fuel is used to cool and lubricate the fuel injectors and fuel injection pump. The fuel return system is designed to return this fuel to the fuel tank. If the high side fuel pressure becomes excessive, the fuel rail pressure relief valve releases the fuel into the fuel return system . The return fuel travels through the diesel fuel conditioning module and the fuel cooler, then to the fuel tank.

A Cranks But Will Not Run or Hard Start symptom may exist if air is being drawn into the fuel injection system due to the following conditions:

Special Tools Required

Observe the Actual Fuel Rail Pressure parameter with a scan tool. During engine cranking, the pressure should be at least 10 MPa. With the engine running at idle the pressure should be close to the Desired Fuel Rail Pressure. As the engine RPM's increase, the Desired Fuel Rail Pressure and the Actual Fuel Rail Pressure should closely match each other.

1. Remove the engine cover. Refer to Engine Cover Replacement .
2. Clean all of the fuel lines between the fuel injection pump and the fuel injectors using brake cleaning solvent, and let dry.

Important: It may be necessary to remove engine components for the visual inspections.

3. Install the CH-48027 to the fuel system service port on the right front side of the engine.

Important: Fuel line connections should be inspected thoroughly for O-ring damage.

4. Command the fuel pump ON with a scan tool and observe the fuel pressure gage to ensure that fuel pressure is present. Visually inspect the engine and chassis for leaks or damage to the fuel hoses, fuel lines, and fuel system components.

⇒	Repair any leaks or replace any components found to be damaged or leaking.

⇒	If the fuel pump did not run and fuel pressure was not observed, refer to Fuel Pump Electrical Circuit Diagnosis .

⇒	If the fuel pump did run and fuel pressure was not observed , inspect the fuel feed line for a vacuum leak between the DFCM and the fuel tank sender.

5. If leaks or damage were not found on the fuel system, turn OFF the fuel pump and disconnect the fuel supply line at the engine. Refer to Metal Collar Quick Connect Fitting Service .
6. Cap off the fuel supply pipe on the engine with a rubber plug and a clamp.

Important: The fuel prime pump must be disabled before using the CH-48027 to diagnose the fuel system. Fuel pressure with the pump running is more than the gage is designed to handle and may cause damage to the gage.

7. Remove the fuel pump relay. Crank the engine and observe the CH-48027 to see if the fuel injection pump is creating at least 10 inches Hg of vacuum.

⇒	If vacuum is present during cranking but bleeds off rapidly, repair the leak in the fuel supply line.

⇒	If vacuum is not present during cranking, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement .

Important: Changes in engine speed can cause an increase of air bubbles that enter the EN-47969 . An air pocket at the top of the EN-47969 loop is considered a normal condition.

8. Remove the rubber plug from the fuel supply line and install the EN-47969 . Create a vertical loop in the hose to observe the incoming fuel for air bubbles. Command the Fuel Pump ON with a scan tool and prime the fuel system until all air has been purged. Start and run the engine for at least 10 minutes to allow the fuel system to stabilize. At a stabilized idle, observe the fuel entering the EN-47969 for air bubbles.

⇒	If air bubbles were observed entering the transparent hose, test the fuel sender for leaks. Refer to Fuel Sender Assembly Replacement .

9. Install the fuel sender, fuel tank, and all fuel system connections and components that were previously disconnected. Leave the EN-47969 connected to the fuel supply line. Command the fuel pump ON with a scan tool and prime the fuel system until all air is purged. Start and run the engine, while observing the fuel entering the transparent hose.

⇒	If air bubbles are still present, refer to Diagnostic Aids.

⇒	If no air bubbles are present , remove the EN-47969 and run the engine to ensure no fuel leaks exist.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.