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P codepowertrainMedium severity

P0519Idle Air Control System Performance

Actual idle speed is deviating from the target idle speed even though the idle control system is being commanded correctly.

Definition

The PCM constantly compares actual engine RPM at idle to its calculated target and adjusts the idle air control valve (or throttle position on some systems) to close the gap. P0519 sets when idle speed remains too high or too low beyond a calibrated limit despite the PCM commanding full correction — a performance fault rather than an electrical one. Symptoms include a high or hunting idle, stalling at stops or when the A/C engages, and an illuminated MIL.

Common causes

  • Carbon buildup restricting the IAC passage or throttle bore
  • Vacuum leak at an intake gasket, hose, or the PCV system raising idle beyond control authority
  • Sticking or worn idle air control valve
  • Throttle plate or stop screw out of adjustment, or a binding throttle cable
  • Leaking or stuck EGR valve upsetting idle quality
  • Incorrect base idle after throttle body service without a relearn

Common misdiagnoses

  • IAC valve replaced when a vacuum leak or carbon is the cause
  • Throttle position sensor
  • MAP sensor
  • PCM

Troubleshooting steps

  1. 1.Compare target and actual RPM

    On a scan tool, note desired idle versus actual idle and the IAC counts or duty cycle; counts pegged at one extreme show the PCM has run out of authority in that direction.

  2. 2.Check for vacuum leaks

    Smoke-test the intake or spray carburetor cleaner around gaskets and hoses; an RPM change or visible smoke locates unmetered air causing high idle.

  3. 3.Clean the throttle body and IAC passage

    Remove carbon from the throttle plate, bore, and IAC pintle passage, which commonly causes low idle and stalling as deposits choke bypass air.

  4. 4.Inspect throttle mechanics

    Verify the throttle plate closes fully against its stop, the cable has correct freeplay, and no aftermarket adjustment has raised the base idle.

  5. 5.Test the IAC valve response

    Command the IAC through its range with a scan tool; idle should sweep smoothly several hundred RPM, and a sluggish or absent response means a sticking valve.

  6. 6.Perform the idle relearn

    After any cleaning or battery disconnect, run the manufacturer's idle relearn procedure and confirm actual idle tracks target hot and cold.

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