GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1996 🢒 Caprice 🢒 Engine 🢒 Engine Controls - 4.3L (Caprice Only) and 5.7L 🢒 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 Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Check PCM grounds for being clean, tight and in their proper locations. Refer to PCM wiring diagrams in Electrical Diagnosis.
    • Remove and check air filter element for dirt, or for being restricted. Refer to Air Filter Element and Air Cleaner Assembly Replace as necessary.

Fuel System
    • Perform injector balance test. Refer to Fuel Injector Balance Test for procedure.
    • Check for low fuel pressure. Refer to Fuel System Diagnosis .
    • Check for restricted fuel filter. Refer to Fuel System Diagnosis .
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Test .
    • Check for incorrect minimum idle speed. Using a scan tool, monitor IAC position. IAC should be between 5 and 50 counts.
    • Check each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2). Relocate injector harnesses as necessary.
    • Check for injectors that are shorted internally. Compare injector resistance. Injector resistance should be within one ohm of each other. Refer to Fuel Injector Solenoid Coil Test .
    • Check for fuel in pressure regulator vacuum hose. If fuel is present, replace regulator assembly. Refer to Fuel Pressure Regulator Replacement .
    • Check for foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft. Also inspect for throttle body tampering.
    • Check items which can cause an engine to run rich (long term fuel trim near -14%). 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 22%). 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 Crankcase ventilation valve for proper operation. Place finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.
    • Using a Scan Tool, monitor TP sensor voltage. The TP sensor voltage should be less then 0.9 volt. If the TP sensor voltage is greater than 0.9 volt, inspect the ASR/TCS adjuster assembly (If so equipped). Also inspect throttle body and accelerator cables for binding. Refer to Adjuster Assembly Throttle And Cruise Control Cables.
    •  Using a scan tool, monitor KS system for excessive spark retard activity.
    • Check the EGR system for proper operation.

Ignition System
    • Check for proper ignition voltage output with spark tester J 26792. Refer to Distributor Ignition (DI) System Check for procedure.
    • Remove spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability problem. Refer to spark plug gap specification is Section OB. The spark plug gapping should be done using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement. If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to DTC P0172 Fuel Trim System Rich Bank 1 and DTC P0175 Fuel Trim System Rich Bank 2 to determine the Rich condition or Engine Mechanical for the oil fouling condition.
    • Visually/Physically inspect Secondary ignition for the following:
       - Ignition wires for cross firing.
       - Ignition wires arcing to ground.
       - Ignition wires for proper routing.
       - Distributor cap for cracks or carbon tracking.
    • Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.
    •  Check for loose ignition coil ground. Refer to Distributor Ignition (DI) System Check
    •  Check distributor vent system for being restricted or open. Refer to Distributor Vent System Check .

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

Refer to Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check exhaust system for possible restrictions. Check for the following:
       - Inspect exhaust system for damaged or collapsed pipes.
       -  Inspect mufflers for heat distress or possible internal failure.
       -  For possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine. Check back pressure by removing AIR check valves near exhaust manifolds. Refer to Restricted Exhaust System Check and Exhaust System.
    • An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits). Check distributor ground circuit for being open or loose.
    • Is Traction Control Adjuster Assembly out of adjustment? Refer to Adjuster Assembly-Throttle And Cruise Control Cables.
    • Check PNP switch circuit . Refer to Park/Neutral Position Switch Check , or use the Tech 1 and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive.
    • Check for faulty motor mounts. Refer to Engine Mechanical for inspection of mounts.
    • Check for intake and exhaust manifold passages for casting flash. Refer to Engine Mechanical.
    • An EGR valve that is excessively coked, could cause an engine misfire. Remove the EGR valve and clean as necessary. Refer to Exhaust Gas Recirculation Valve Replacement .