GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

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

Preliminary Check

Refer to Symptoms .

Sensor Checks
    • Check the Heated Oxygen Sensor (HO2S).
        Check for silicon contamination from fuel or improperly used sealant. The sensor will have a white powdery coating. The sensor will result in a high but false signal voltage (rich exhaust indication). The Control Module will reduce the amount of fuel delivered to the engine causing a severe driveability problem.
    • Check the Throttle Position (TP) sensor. If a sticking throttle shaft or binding linkage causes a high TP sensor open throttle indication, the Control Module will not control the idle. Monitor the TP sensor voltage. A scan tool and/or voltmeter should read less than 0.85 volts with the throttle closed.
    • Check the Engine Coolant Temperature (ECT) sensor using the scan tool to compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees greater than or less than the ambient air temperature on a cold engine, check for a high resistance in the coolant sensor circuit or the sensor itself.
    • Check the Manifold Absolute Pressure (MAP) sensor response and accuracy.

Fuel System Checks
    • Check the fuel pressure. Refer to Fuel System Diagnosis .
    • Check to determine if a Rich or Lean system causes the condition. Drive the vehicle at the speed of the complaint. Monitoring the Fuel Trim will help identify the problem.
       - Lean - The Long Term Fuel Trim is greater than 150.
       - Rich - The Long Term Fuel Trim is less than 115.
    • Perform the fuel injector balance test. Refer to Fuel Injector Balance Test .
    • Check the fuel injector driver circuit.
        1. Disconnect the injector harness connector at the injectors.
        2. Connect an injector test light between the terminals of each injector connector and note the light while cranking.
        3. If the test light fails to blink at any connector, it is a poor injector drive circuit harness, connector, or terminal.
    • Perform the fuel injector coil test. Refer to Fuel Injector Solenoid Coil Test .
    • Check for fuel in the pressure regulator vacuum hose.
    • Check the Evaporative Emission (EVAP) control system. Refer to Knock Sensor (KS) System Diagnosis .

Ignition System Checks
  1. Check the ignition output using the J 26792 spark tester or the equivalent. Refer to Enhanced Ignition System Diagnosis .
  2. Check the following conditions:
    3. • Wet plugs
    • Cracks
    • Wear
    • Improper gap
    • Burned electrodes
    • Blistered insulators
    • Heavy deposits
  3. Check the spark plugs wires by connecting a J 39200 DMM to the ends of each wire in question. If the meter reads over 30,000 ohms, then replace the wires.

Additional Checks
    • Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle and low Idle Air Control (IAC) counts.
    • Check the IAC operation. Refer to Idle Air Control (IAC) System Diagnosis .
    • Check the Control Module grounds for being clean, tight, and in their proper locations. Refer to Engine Controls Component Location.
    • Check the scan tool to determine if the Control Module is receiving an A/C signal. Refer to Air Conditioning (A/C) Compressor Control Circuit Diagnosis . If a problem exists with the A/C On check, the A/C system operation.
    • Check for the EGR being ON while idling which will cause roughness, stalling, and hard starting. Refer to Exhaust Gas Recirculation (EGR) System Diagnosis .
    • Check the battery cables and ground straps. They should be clean and secure.
    • Check the Crankcase Ventilation Valve for proper operation by placing a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve. Refer to Crankcase Ventilation System Diagnosis .

Engine Mechanical Check

Check for the following:

    • Broken motor mounts.
    • Improper valve timing
    • Low compression
    • Bent pushrods
    • Worn rocker arms
    • Broken or weak valve springs.
    • Worn camshaft lobe.