GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Sierra - 4WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 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 .
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight and in the proper locations. Refer to Engine Controls Schematics .
    • Remove and check the air filter element for dirt, or for being restricted. Refer to Air Cleaner Element Replacement . Replace as necessary.

Fuel System
    • Check for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Check for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Check for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check that each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-7-2-6-5-4-3). Relocate injector harnesses as necessary.
    • Check items that cause an engine to run rich (long term fuel trim near -13%). For a rich condition, refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 and DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1 .
    • Check items that can cause an engine to run lean (long term fuel trim near 24%). For a lean condition, refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 and DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1 .

Sensor/System
    • Check the crankcase ventilation valve for proper operation. Place a finger over the inlet hole of the valve end several times. The valve should snap back. If not, replace the valve.
    •  Use a scan tool in order to monitor KS system for excessive spark retard activity.

Ignition System
    • Check for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis for procedure.
    • Remove spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits

Refer to Spark Plug Visual Diagnosis in Engine Electrical.

    • An improper spark plug gap will cause a driveability problem. Refer to Spark Plug Visual Diagnosis in Engine Electrical. Gap the spark plugs using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement in Engine Electrical.
    • Determine the cause of the fouling before replacing the spark plugs if the spark plugs are gas, coolant, or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 or DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition. Refer to Spark Plug Visual Diagnosis in Engine Electrical for diagnosis of coolant or oil fouled spark plugs.
    • Visually/Physically inspect Secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as you apply water.
    • Monitor the Misfire Current Counters while driving the vehicle in the conditions that the misfire occurred. If a cylinder can be located with a misfire use the DTC P0300 table for diagnosis.
    •  Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Mechanical
    • Check engine mechanical for the following:
       - Check compression. Refer to Engine Compression Test in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Sticking or leaking valves. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Worn camshaft lobe(s). Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Valve timing. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Bent push rods. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Worn rocker arms. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Broken valve springs. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Low cylinder compression. Refer to Engine Compression Test in Engine Mechanical 4.8L, 5.3L, 6.0L.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Engine/Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check the exhaust system for possible restrictions. Check for the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the exhaust manifold for a collapsed inner wall.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine. Check the back pressure by removing AIR check valves near exhaust manifolds. Refer to Restricted Exhaust in Engine Exhaust.
    • 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 secondary ignition wires or high voltage components near ignition control circuits.
    • Check the PNP switch circuit. Refer to DTC P0705 Trans Range Switch Circuit or DTC P0706 Trans Range Switch Performance .
    • Check for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical 4.8L, 5.3L, 6.0L.
    • Check the intake manifold and the exhaust manifold passages for casting flash. Refer to Engine/Engine Mechanical.
    • Check for an Exhaust Gas Recirculation (EGR) valve stuck open or EGR pintle not seating properly. Inspect EGR valve gasket for leaks.