GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Engine 🢒 Engine Controls and Fuel - 7.8L (LG4) 🢒 Fuel System Diagnosis

Description

The common rail fuel system is comprised of 2 fuel pressure sections, a suction side between the fuel tank and the fuel supply pump, and a high pressure side between the fuel supply pump and the fuel injectors. Fuel is drawn from the fuel tank via a feed pump and then pumped into the fuel rail by 2 plungers, all of which are internal to the fuel supply pump. This high pressure is regulated by the engine control module (ECM) using the fuel rail pressure (FRP) regulator dependant upon values from the FRP sensor attached to the fuel rail. In case of fuel rail overpressure, a pressure limiter valve threaded into the fuel rail will open to release overpressure and return fuel back to the fuel tank.

The Fuel System Check diagnostic table directs the service technician to the appropriate fuel system diagnosis. The diagnostic table assumes the following conditions are met:

    • The batteries are completely charged.
    • The engine cranking speed is normal.
    • There is adequate fuel in the fuel tank.
    • There is no air in the fuel line.

Diagnostic Aids
    • The fuel system, from the fuel tanks to the fuel supply pump is under a slight vacuum with the engine running. As a result, air can enter the fuel system if these connections are not tight or if there is a crack in one of the fuel hoses. Air in the fuel system will cause fuel rail pressure fluctuations, especially at high engine speed and load.
    • If the fuel tank is empty or near empty, air might be allowed to go into the fuel system. With air in the fuel system, smooth flow of the fuel into the supply pump is interrupted. Perform bleeding of the fuel system after refilling.
    • The Actual Fuel Rail Pressure on the scan tool will only update with the engine running, or one time after the key is cycled when the ignition has been turned OFF for longer than 10 seconds.

Step

Action

Values

Yes

No

1
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the DTC information of the engine control module (ECM) with a scan tool.

Are any DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 2

2

    Important: Fuel may leak under the cylinder head cover from the inlet high pressure line. In such case, the engine oil level will rise. Inspect for fuel in the engine oil.

  1. Inspect the high pressure side between the fuel supply pump and the fuel injectors for fuel leakage. The following components may contain an external leak:
    2. • The fuel supply pump
    • The fuel rail
    • The pressure limiter valve
    • The flow damper valve
    • The fuel rail pressure (FRP) sensor
    • The fuel pipe between the fuel supply pump and fuel rail
    • The fuel pipe between the fuel rail and fuel injectors
    • Each fuel pipe sleeve nut
  2. Repair any fuel system leaks as necessary.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 3

3
  1. Turn OFF the ignition.

    2. Important: The fuel system, from the fuel tanks to the fuel supply pump, is under a slight vacuum with the engine running. As a result, air can enter the fuel system if these connections are not tight. Air in the fuel system will cause fuel rail pressure fluctuations, especially at high engine speed and load.

  2. Inspect the fuel system line connections between the fuel tank and the fuel supply pump for tightness and all fuel hoses for cuts, cracks, and for the use of proper clamps.
  3. Pump the priming pump on the fuel filter until it becomes firm. If there is a leak on the suction side of the fuel system between the priming pump and the fuel supply pump, the priming pump will not build up sufficient firmness and fuel leakage may occur.
  4. Repair any fuel system leaks as necessary.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 4

4
  1. Ensure the fuel tanks have adequate fuel and the fuel quality is good. Take a sample.
  2. Bleed the air from the fuel system as necessary. Refer to Fuel System Bleeding .
  3. Start the engine and let idle. If the engine does not start continue to crank.
  4. Observe the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure parameters with a scan tool.

Is the difference between the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure less than the specified value?

725 psi (5 MPa)

System OK

Go to Step 5

5
  1. Turn OFF the ignition.
  2. Disconnect the inline harness connector from the cylinder head cover case.
  3. Remove the rubber fuel hose from the fuel filter housing, fuel supply pump side. Use a pan to catch the fuel leakage from the removed fuel line.

    4. Important: The fuel vacuum pump/pressure gage connector and the adapter hose must be cleaned before connecting to the fuel line. Foreign material internal to the tools line may damage the fuel supply pump.

  4. Connect the suction side fuel pressure adapter EN-47667 with J 44638 Vacuum Gage, in series, with the filter housing and the disconnected fuel hose. Ensure the service tool and fuel line connections are tight.
  5. Bleed the fuel system by priming the priming pump until it becomes firm, then crank over the engine for a maximum of 5 seconds. Perform procedure 3 times.
  6. Fully clamp off a fuel hose as close to the fuel tank as possible, this will draw vacuum on the fuel system. You can also disconnect a fuel line and plug it.

    7. Important: Release the clamp or open the plug when the gage is likely to be more than 8 in Hg during the test.

  7. Crank over the engine for less than 15 seconds, while monitoring the J 44638 .

Can a vacuum of at least the specified amount be pulled on the fuel system?

8 in Hg

Go to Step 8

Go to Step 6

6
  1. Inspect the fuel system line connections between the fuel tank and the fuel supply pump for tightness and all fuel hoses for cuts, cracks, and for the use of proper clamps.
  2. Draw fuel from the fuel tank at the fuel line going to the fuel pickup tube to verify a clean stream of fuel comes out. Use the J 23738-A Mityvac with a clear hose or equivalent. This will ensure the fuel pickup tube is not cracked drawing air into the fuel line.
  3. Repair as necessary.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 7

7
  1. Replace the fuel filter cartridge. Refer to Fuel Filter Cartridge Replacement.
  2. Retest the fuel system vacuum test described in step 5.

Can a vacuum of at least the specified amount be pulled on the fuel system?

8 in Hg

Go to Step 18

Go to Step 17

8
  1. Unclamp or unplug the fuel line from step 5 and reconnect the fuel line.
  2. Prime the priming pump until firmness is felt.
  3. Crank over the engine for 5 seconds.
  4. Repeat steps 2 and 3, 3 times to bleed the fuel system.
  5. Crank over the engine for less than 15 seconds at a time, while monitoring the J 44638 .

Does the J 44638 ever indicate a value larger than the specified value?

5 in Hg

Go to Step 9

Go to Step 10

9
  1. Inspect the fuel lines between the fuel supply pump and the fuel tank for being crushed or kinked.
  2. Inspect the fuel cap for a plugged condition.
  3. Inspect inside the fuel tank, if possible, for any foreign material that may be getting drawn into the fuel line pickup causing a blocked condition.
  4. Repair the condition as necessary.

Did you find and correct the condition?

--

Go to Step 18

Go to Step 14

10

Important: Perform this test 3 times and take the average of the fuel return amount coming from the cylinder head.

  1. Turn OFF the ignition.
  2. Ensure the inline harness connector is disconnected from the cylinder head cover case.
  3. Remove the eyebolt from the cylinder head that connects to the leak-off pipe.
  4. Connect the J 45873-103 Fuel Adapter with hose assembly to the cylinder head. Install the hose into a graduated cylinder from the J 45873-100 kit.
  5. Remove the eyebolt from the pressure limiter valve that connects to the leak-off pipe.
  6. Connect the J 45873-105 Fuel Adapter with hose assembly to the pressure limiter valve.
  7. Crank over the engine for 10 seconds. Ensure the batteries are fully charged and the engine is cranking at normal speed for each test.

Is fuel leaking from the pressure limiter valve?

--

Go to Step 15

Go to Step 11

11

Is the average fuel return coming from the cylinder head less than the specified value?

30 ml

Go to Step 12

Go to Step 13

12
  1. Remove each glow plug from the cylinder head.
  2. Inspect the tip of the plugs for wet fuel.

Did you find wet fuel on any glow plug?

--

Go to Step 16

Go to Step 17

13
  1. Remove the cylinder head cover and injector harness in order to connect the J 45873-101 with hose assembly to each injector leak off port. Refer to Fuel Injector Replacement .
  2. Crank over the engine for 10 seconds.

    3. Important: Replacement injector must be programmed.

  3. Replace the injectors that return more fuel than the specified value. Refer to Fuel Injector Replacement .
  4. Retest after replacement of the fuel injectors.

Did you complete the replacement?

3 ml

Go to Step 18

--

14

Replace the fuel filter cartridge. Refer to Fuel Filter Replacement .

Did you complete the replacement?

--

Go to Step 18

--

15

Replace the pressure limiter valve. Refer to Fuel Injection Fuel Rail Assembly Replacement .

Did you complete the replacement?

--

Go to Step 18

--

16

Important: Replacement injector must be programmed.

Replace the appropriate injectors that was leaking fuel found at step. Refer to Fuel Filter Replacement .

Did you complete the replacement?

--

Go to Step 18

--

17

Important: The fuel supply pump must be timed to the engine and adjustment value must be learned to the ECM.

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

Did you complete the replacement?

--

Go to Step 18

--

18
  1. Reconnect all previously disconnected harness connectors or components.
  2. Turn OFF the ignition for 30 seconds.
  3. Bleed the air from the fuel system. Refer to Fuel System Bleeding .
  4. Start the engine and let idle.
  5. Observe the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure parameter with a scan tool.

Is the difference between the Actual Fuel Rail Pressure and Desired Fuel Rail Pressure less than the specified value?

725 psi (5 MPa)

System OK

Go to Step 1