GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 8.1L 🢒 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.
    • Check the powertrain control module (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 the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • 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: 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 percent. For a rich condition, refer to DTC P0132 or P0152 .
    • Check items that can cause an engine to run lean long term fuel trim near 24 percent. For a lean condition, refer to DTC P0131 or P0151 .

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 the knock sensor (KS) system for excessive spark retard activity.

Ignition System
    • Check for proper ignition voltage output with J 26792 Spark Tester . 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 Inspection .
    • An improper spark plug gap will cause a driveability problem. Gap the spark plugs using a wire gage gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs if the spark plugs are gas, coolant, or oil fouled. Refer to Spark Plug Inspection for diagnosis of fouled spark plugs.
    • Visually and physically inspect the secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition wires for proper routing
    • Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply water.
    • 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 .
       - 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. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • Inspect the following components for incorrect basic engine parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Refer to Symptoms - Engine Mechanical in Engine Mechanical.

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
       - The three-way catalytic converters for possible plugged conditions--Compare the exhaust system back pressure on each side of engine.
    • 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 the ignition control circuits.
    • Check the park neutral position (PNP) switch circuit.
    • Check for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical .
    • Check the intake manifold and the exhaust manifold passages for casting flash.
    • Check for an exhaust gas recirculation (EGR) valve stuck open or EGR pintle not seating properly. Inspect EGR valve gasket for leaks.