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P codepowertrainMedium severity

P2279Intake Air System Leak

The PCM has detected unmetered air entering the engine through a leak in the intake air system.

Definition

The PCM cross-checks measured airflow (MAF), manifold pressure (MAP), throttle position, and idle-control airflow against each other; when more air reaches the cylinders than the throttle position and MAF reading can account for, it sets P2279 for an intake air system leak. The code is especially common on Honda and Acura vehicles. Symptoms include high or unstable idle, lean fuel trims, hesitation, and an illuminated MIL, and the underlying lean condition can trigger additional fuel-trim codes and an emissions-test failure.

Common causes

  • Leaking intake manifold or throttle body gasket
  • Cracked, loose, or disconnected vacuum hoses (including PCV and brake booster lines)
  • Torn intake duct or loose clamps between the MAF and throttle body
  • Heavy carbon deposits in the throttle body altering idle airflow calibration
  • Brake booster diaphragm leaking vacuum internally
  • Missing or leaking oil filler cap or dipstick seal on some designs

Common misdiagnoses

  • MAF sensor replaced when the air is simply bypassing it
  • Oxygen sensors replaced for the resulting lean readings
  • Idle air control valve or throttle body replaced before finding the leak
  • Fuel pump replaced chasing lean fuel trims

Troubleshooting steps

  1. 1.Read fuel trim data at idle and cruise

    Long-term trims strongly positive at idle (over +10%) that normalize at higher rpm are the classic signature of a vacuum leak downstream of the throttle.

  2. 2.Visually inspect all intake ducts, hoses, and clamps

    Flex each rubber duct to expose hidden cracks, and confirm the PCV hose, brake booster hose, and any EVAP purge lines are connected and intact.

  3. 3.Smoke-test the intake system

    Introduce smoke through a vacuum port with the throttle sealed and watch for smoke escaping at gaskets, hose connections, and the intake manifold seams.

  4. 4.Test suspect areas with carb cleaner or propane

    With the engine idling, an rpm change when spraying around the manifold gasket or a hose junction pinpoints the leak.

  5. 5.Clean the throttle body and perform any required idle relearn

    On Honda applications especially, clean carbon from the throttle bore and plate, then complete the idle-learn procedure per service information.

  6. 6.Repair the leak, clear codes, and verify

    Confirm fuel trims return to roughly ±5% at idle and the code does not reset over a full drive cycle.

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