GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 TopKick C-Series (Conventional) C6/C7/C8 🢒 Engine 🢒 Engine Controls - 7.8L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

DEFINITION: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary Checks
    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Inspect the engine control module (ECM) grounds for being clean, tight, and in the proper locations.
    • Remove and inspect the air filter element for dirt or for being restricted. Replace as necessary.

Fuel System
    • Inspect for a leak in the fuel supply system.
    • Inspect the fuel injectors.
    • Inspect for a restricted fuel filter.
    • Inspect for a restriction in the fuel return system.
    • Inspect for a contaminated fuel condition. Refer to Contamination Testing .

Sensor/System
    • Inspect for proper crankcase ventilation.
    • Inspect for an intermittent crankshaft position (CKP) sensor signal by monitoring the RPM with a scan tool.
    •  Use a scan tool in order to monitor the sensor system for unusual sensor readings. Refer to Scan Tool Data List .

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Poor cylinder compression--2157 kPa (313 psi).
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber--Leaking valve seals.
    • Inspect the following components for incorrect basic engine parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Inspect the following components of the exhaust system for possible restrictions:
       - The exhaust system for damaged or collapsed pipes
       - The exhaust manifold for a collapsed inner wall
       - The mufflers for heat distress or possible internal failure
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change indicates that EMI is present. If a problem exists, check routing of high voltage components, such as fuel injector wiring, near the sensor circuits.
    • Inspect the park neutral position (PNP) switch circuit.
    • Inspect for faulty motor mounts.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.