GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L (LLY) 🢒 Fuel System Diagnosis - High Pressure Side

Supply System

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 hand prime pump and a filter element. Within the assembly, there is also a fuel heater. The 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.

High Pressure System

The much larger section of the fuel injection pump assembly is the high-pressure pump. The pump is engine-driven by the camshaft gear. From the high-pressure pump, the fuel flows to the 2 common fuel rails. Each common fuel rail supplies one bank of 4 fuel injectors.

Return System

The fuel return system routes fuel from the fuel injectors, the pressure relief valve, 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.

Diagnostic Aids
    • The fuel return volumes vary based on the American Petroleum Institute (API) rating of the diesel fuel.
    • A fuel injector may have high fuel return flow only at higher engine temperatures.

Step

Action

Yes

No

1

Were you referred to this test from the Engine Cranks but Does Not Run diagnostic?

Go to Step 3

Go to Step 2

2

Important: Fuel that is contaminated with gasoline may cause permanent damage to the fuel pressure relief valve. Refer to Contaminants-in-Fuel Diagnosis .

  1. Remove the fuel return rubber hose from the rear of the left fuel rail and plug the hose to prevent fuel leakage.
  2. Install a section of rubber fuel hose on the fuel pressure relief valve connection and place it in a clean fuel container.
  3. Start and idle the engine.
  4. Command the fuel rail pressure to 160 MPa with a scan tool.
  5. Observe the fuel hose and container for fuel leakage.

Did fuel flow into the container?

Go to Step 12

Go to Step 4

3

Important: If you were not referred to this test from another diagnostic, do not perform this procedure.

Only perform this test when the fuel is more than 18°C (65°F).

  1. Remove the air duct from the air cleaner assembly and the turbo inlet.
  2. Remove the fuel return rubber hose from the rear of the left fuel rail and plug the hose to prevent fuel leakage.
  3. Remove the relay that supplies ignition 1 voltage to the fuel injection control module (FICM).
  4. Crank the engine for 15 seconds while observing the fuel pressure relief valve hose connection for fuel leakage.

Did fuel leak from the fuel pressure relief valve at the hose connection?

Go to Step 12

Go to Step 4

4

Important: The engine cranking speed must be more than 150 RPM during the cranking portion of this test.

Always replace the fuel return hose retaining clips on the fuel injectors with new clips after removing.

  1. Remove the relay that supplies the ignition 1 voltage to the FICM, if it was not previously removed.
  2. Remove the fuel return hose from the fuel injectors of the right cylinder bank. Refer to Fuel Injection Fuel Return Pipe Replacement - Right Side .
  3. Connect the yellow hoses from the J 45873 Fuel Return Volume Test Kit to the J 45873-30 Fuel Injector Test Adapters.
  4. Connect the J 45873-30 with yellow hoses to each fuel injector return port of the right cylinder bank, and install the retaining clips.
  5. Install the 4 yellow hoses in the J 45873 graduated cylinders in numerical order.
  6. Connect the fuel return hoses to the J 45873-30 to prevent leakage.
  7. Crank the engine in 15-second intervals, with 1 minute cooling time between intervals, until fuel starts to flow into all of the graduated cylinders.
  8. Elevate the 4 hoses to retain the fuel in the hoses, and empty the 4 graduated cylinders into a suitable container.
  9. Install the 4 yellow hoses in the J 45873 graduated cylinders in numerical order.
  10. Crank the engine for 15 seconds.
  11. Measure the quantity of fuel in each of the graduated cylinders. Refer to Fuel System Specifications for the desired quantities.

Were any of the fuel quantities more than the specified values?

Go to Step 5

Go to Step 6

5
  1. Remove the fuel feed pipes from any fuel injectors that have high return flow and install the EN 47589 Fuel Pressure Adapter Cap to the fuel rail outlets.
  2. Elevate the 4 hoses to retain the fuel in the hoses, and empty the 4 graduated cylinders into a suitable container.
  3. Install the 4 yellow hoses in the graduated cylinders in numerical order.
  4. Crank the engine for 15 seconds.
  5. Using the Retesting Fuel Injector Return Flow Values table, measure the quantity of fuel in each of the graduated cylinders. Refer to Fuel System Specifications for the desired quantities.
  6. Remove the fuel feed pipes from any uncapped fuel injectors that have high return flow and install the EN 47589 to the fuel rail outlets.

Did you complete the action?

Go to Step 6

--

6

Important: The engine cranking speed must be more than 150 RPM during the cranking portion of this test. Always replace the fuel injector return hose retaining clip.

  1. Remove the fuel return hoses from the fuel injectors of the left cylinder bank. Refer to Fuel Injection Fuel Return Pipe Replacement - Left Side .
  2. Connect the yellow hoses from the to the J 45873-30.
  3. Connect the J 45873-30 with yellow hoses to each fuel injector return port of the left cylinder bank, and install the retaining clips.
  4. Connect the fuel return hoses to the J 45873-30 to prevent leakage.
  5. Crank the engine in 15-second intervals, with 1 minute cooling time between intervals , until fuel starts to flow into all of the graduated cylinders.
  6. Elevate the 4 hoses to retain the fuel in the hoses, and empty the 4 graduated cylinders into a suitable container.
  7. Install the 4 yellow hoses in the graduated cylinders in numerical order.
  8. Crank the engine for 15 seconds.
  9. Measure the quantity of fuel in each of the graduated cylinders. Refer to Fuel System Specifications for the desired quantities.

Were any of the fuel quantities more than the specified values?

Go to Step 8

Go to Step 7

7

Were any fuel injector lines capped off during either cylinder bank testing?

Go to Step 10

Go to Step 11

8
  1. Remove the fuel feed pipes from any fuel injectors that have high return flow and install the EN 47589 to the fuel rail outlets.
  2. Elevate the 4 hoses to retain the fuel in the hoses, and empty the 4 graduated cylinders into a suitable container.
  3. Install the 4 yellow hoses in the graduated cylinders in numerical order.
  4. Crank the engine for 15 seconds.
  5. Using the Retesting Fuel Injector Return Flow Valves table, measure the quantity of fuel in each of the graduated cylinders. Refer to Fuel System Specifications for the desired quantities.

Were the fuel quantities more than the specified value at the uncapped fuel injectors?

Go to Step 9

Go to Step 10

9

Repeat the fuel injector return flow test with capping until no fuel injectors have high return fuel flow.

Did you complete the action?

Go to Step 10

--

10

Important: If the engine does not start after hand priming the fuel system, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement .

  1. Replace the fuel injectors that had capped off lines. Refer to Fuel Injector Replacement .
  2. Install all disconnected and removed fuel system components.
  3. Start and idle the engine. You may have to prime the fuel system before the engine will start.
  4. Command the fuel pressure control to 160 MPa with a scan tool.

Is the fuel pressure more than 145 MPa?

Go to Step 13

Go to Step 11

11

Replace the fuel injection pump. Refer to Fuel Injection Pump Replacement .

Did you complete the replacement?

Go to Step 13

--

12

Replace the fuel pressure relief valve. Refer to Fuel Pressure Relief Valve Replacement .

Did you complete the replacement?

Go to Step 13

--

13

Important: The fuel system may have to be primed before starting the engine.

  1. Install and connected all fuel system components that were previously removed or disconnected.
  2. Operate the vehicle in order to verify the repairs.

Did you correct the condition?

System OK

Go to Symptoms - Engine Controls