Fuel is drawn by the fuel injection pump through a pre-filter screen in the tank and to the engine through the fuel supply lines. Fuel flows to the fuel filter/heater element housing , which combines a water separator, a hand prime pump, fuel heater element and a filter element. The fuel is then delivered to the high-pressure pump. The integrated hand 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 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 rail then feeds the right fuel rail. Each fuel rail supplies one bank of 4 fuel injectors. The fuel rail pressure sensor is mounted in the middle of 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 fuel cooler and 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.

Important: Ensure that a sufficient amount of fuel is in the fuel tank to run the vehicle.

1. Install the CH-48027 to the fuel system service port on the right front side of the engine.
2. Prime the fuel system until 10 psi is indicated on the CH-48027 .

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

3. Visually inspect the following items for restrictions or leaks:

•	Hoses and lines that are flattening when the engine is running, or have kinks that would restrict the flow of fuel

•	Fuel leaks between the fuel filter/heater element housing and the fuel injection pump

4. Repair any leaks or replace any components found to be damaged or leaking.
5. Disconnect the fuel supply line at the engine and cap off the pipe on the engine with a rubber plug and clamp. Refer to Metal Collar Quick Connect Fitting Service .
6. Disconnect the fuel return pipe and install a J 23738-A .
7. Apply 25  inches Hg of vacuum to the fuel return pipe and wait for at least one minute for the system to stabilize. Repeat this step 6 additional times.

⇒	If vacuum drops below 8 inches Hg of vacuum within 10 minutes, repair the air leak at fuel filter/heater element housing connections or at the fuel hoses/pipes, on the engine.

Important: The engine may start, and then stall after briefly running.

8. Connect the fuel return pipe and crank the engine 2-3 times in 15-second intervals while observing the CH-48027 . More than 5 inches Hg vacuum should be observed during cranking.

⇒	If less than 5 inches Hg of vacuum was observed on the vacuum gage during cranking, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement .

9. Disconnect and plug the chassis fuel supply line at the fuel tank. Refer to Metal Collar Quick Connect Fitting Service .
10. Apply at least 25 inches Hg of vacuum to the supply hose with the J 23738-A and wait more than one minute for the system to stabilize. Repeat this action 10 times.

⇒	If the vacuum drops below 8 inches Hg of vacuum within 10 minutes, repair the air leak in the fuel supply line between the engine and fuel tank.

11. Except for the fuel supply line, install all components and lines that were previously removed or disconnected.
12. Install the EN-47969 on the fuel supply line at the engine. Create a vertical loop in the hose to observe the incoming fuel for air bubbles.
13. Prime the fuel system 30 times to remove the air from the fuel hoses. Continue to prime the fuel system until the engine starts and runs.
14. Run the engine for at least 10 minutes to allow the fuel system to stabilize, and purge any air from the system.

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.

15. 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 .

16. 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. Prime the fuel system 30 times to remove the air from the fuel hoses. Continue to prime the fuel system until the engine starts and runs. Observe 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.