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<H1>Fuel System Diagnosis - High Pressure Side</H1><A name=ss1-743071><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=743071&pubid=210&cellId=67449#ss1-743071" target="_blank">
<H5>Supply System</A></H5>

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. 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.<A name=ss2-743071><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=743071&pubid=210&cellId=67449#ss2-743071" target="_blank">
<H5>High Pressure System</A></H5>

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. <A name=ss3-743071><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=743071&pubid=210&cellId=67449#ss3-743071" target="_blank">
<H5>Return System</A></H5>

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.

<LI>If you were not referred to this test from another diagnostic, do not perform this procedure.
<LI>Only perform this test when the fuel is more than 18°C (65°F).
<LI>The fuel return volumes vary based on the API rating of the diesel fuel.
<LI>A fuel injector may have high fuel return flow only at higher engine temperatures. </LI>[/list]

  1. <LI =1>Were you Referred here from one of the following diagnostics?
    <UL =BULLET>
    <LI>DTC P0094
    <LI>Surges/Chuggles in Symptoms
    <LI>Rough, Unstable, or Incorrect Idle in Symptoms
    <LI>Fuel Injector Balance Test with Tech 2
    <UL =SQUARE>
    <LI>If you answered Yes, go to the Fuel Pressure Regulator Graphing after step 48.
    <LI>If you answered No, go to Step 2. </LI>
<LI =1>Remove the air duct from the air cleaner assembly and the turbo inlet.
<LI =1>Remove the air intake pipe. Refer to Air Intake Pipe Replacement .
<LI =1>Drain the coolant and remove the water outlet tube. Refer to Water Outlet Tube Replacement in Engine Cooling.

<LI =1>Remove the fuel injection pump fuel return rubber hose from the junction block (2). Cap the fitting on the block with a 3/8 inch rubber cap (1) to prevent fuel leakage.

<A href="http://service.gm.com/servlets/RetrieveTif?pic=744234" ta
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