GM 6.5L V8 – Fuel System – Rough Idle Diagnosis Check

1. Check fuel injection pump static timing:
• Align the front cover and injection pump marks.

2. Check low idle speed (refer to page 7-87 for the complete procedure steps):
• Adjust to the vehicle emission label specification.

3. Inspect fuel injection lines and nozzles for fuel leakage:
• Repair any leakage.

4. Check fuel supply to the injection pump (refer to page 7-68 for the complete procedure steps):
• Minimum volume is 0.24 liter (1/2 pint) in 15 seconds.
• Minimum pressure is 40 kPa (5.8 psi).
• Fuel must be free of air bubbles.
• Make repairs, if needed.

5. Check the fuel return from the injection pump (refer to page 7-76 for the complete procedure steps):
• Fuel must be free of air bubbles.
• Repair injection pump, if needed.

6. Check each nozzle during engine operation:
• Loosen the injection line fitting nut at the nozzle to be tested.
• Observe that solid fuel flows from the line (no air bubbles).
• If air bubbles exist, go to step 7.
• If air bubbles do not exist, go to step 8.

7. Check a nozzle during engine cranking:
• Remove the B+ wire from the injection pump fuel shut-off solenoid.
• Remove and relocate the injection line to observe the nozzle inlet.
• Crank the engine and watch for air bubbles at the nozzle inlet fitting.
• If bubbles exist, replace the nozzle and go to step 8.
• Install all injection lines and remove all glow plugs.
• Crank the engine while observing the fuel mist exiting each glow plug hole.
• If no mist is present at any of the glow plug holes, remove and test the related injection nozzle.

8. Follow the steps of the glow plug resistance check (see next page).


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