* Increased Engine Load: The AC compressor requires significant power to operate. This added load puts extra strain on the engine, especially at idle when the engine is already running at a lower RPM and producing less power. If the engine is already weak or has other underlying problems, this added load can be enough to cause the idle to become rough or even stall.
* Electrical Demand: The AC compressor is driven by an electromagnetic clutch. Engaging this clutch places a significant demand on the electrical system. If the alternator is weak or the battery is low, this additional electrical draw can affect the engine's performance, leading to an unstable idle.
* Vacuum Leaks: The engine's vacuum system plays a crucial role in various functions, including idle control. A vacuum leak can worsen the effect of the added AC load, making the idle even rougher. The added load might expose a small leak that's otherwise not noticeable.
* Dirty Air Filter: A clogged air filter restricts airflow to the engine, reducing its ability to breathe efficiently. This can exacerbate the problems caused by the added load of the AC compressor.
* Low Refrigerant: While less directly related to the idle, a low refrigerant charge in the AC system can cause the compressor to work harder, increasing the load on the engine and contributing to a rough idle.
* Worn or Failing Engine Components: Problems like worn spark plugs, failing ignition coils, a dirty or failing throttle body, or a problem with the idle air control valve (IACV) can be magnified when the AC is on, leading to a rough idle. These components are already working harder at idle, and the additional load from the AC pushes them over the edge.
* Faulty AC Compressor: Although less likely to directly cause a rough idle, a faulty AC compressor that's struggling to engage or disengage properly can indirectly contribute to the problem.
In short, the AC compressor demands extra power from the engine, and if the engine or related systems are not in optimal condition, this extra load will manifest as a rough idle. A mechanic can diagnose the specific cause by testing the engine's components and electrical system.
