What Causes the Engine RPM to Fluctuate at Idle When the AC is On?

[MacLogan]

Here are the most likely reasons for engine RPM fluctuations at idle when the AC is on: 1. Excessive Carbon Deposits: Carbon buildup in the engine is the primary cause of fluctuating RPM. When there is too much carbon buildup, the fuel injected by the cold-start injector can be largely absorbed by the deposits, resulting in an overly lean air-fuel mixture during startup, which may cause sudden stalling. The solution is to clean the fuel system, inspect the idle control valve, and clean the throttle body. 2. Ignition System Malfunction: Spark plugs are wear-and-tear components that require regular replacement. If spark plugs are excessively carbon-fouled or have an overly large gap, the increased load from the AC can cause RPM fluctuations. 3. Fuel Pressure Issues: Check whether the fuel supply pressure and intake pressure sensors are functioning properly. Abnormal fuel pump pressure or faulty readings from the intake pressure sensor can lead to RPM fluctuations or even engine stalling.