What Causes Unstable Idle When the Air Conditioner is On?
3 Answers
Unstable idle occurs only when the air conditioner is on, and the possible reasons are as follows: 1. Engine control logic error: The system fails to receive the signal indicating the air conditioner is on. When unable to determine whether the air conditioner is activated, the Engine Control Unit (ECU) mistakenly interprets it as a temporary high load and attempts to compensate for the resistance at idle by increasing the engine speed. When the engine speed drops, it can no longer bear the load from the air conditioner compressor, causing the speed to increase again. 2. Air conditioner control circuit error: The system fails to send the air conditioner activation signal. Since the ECU cannot determine the air conditioner's status, it continuously adjusts the idle speed, resulting in unstable idle. 3. Air conditioner circuit malfunction: This causes the air conditioner to work intermittently. When the air conditioner is off, the compressor belt runs without load, so it does not affect the engine load.
When the air conditioning is on, the idle speed becomes unstable. I found this is related to the additional load on the engine. Once the AC starts, the compressor increases the engine load, which may cause RPM fluctuations. Simply put, the engine struggles: if the fuel supply system is clogged or the fuel injectors have carbon buildup, fuel supply becomes insufficient; if the spark plugs are worn out or the ignition coils have issues, uneven ignition worsens the shaking; a dirty intake tract leads to insufficient air, and a faulty idle control valve is also a common cause. I once experienced a slight vacuum line leak, causing the RPM to fluctuate up and down, and eventually spent some money to fix it at the repair shop. Remember, the engine control unit sometimes needs recalibration to adapt to the AC load changes. To avoid this issue during daily driving, regular checks of these components can prevent it.
As a seasoned driver with over a decade of experience, I often encounter idle instability issues when using the AC in summer. The RPM suddenly surges or drops, especially at traffic lights, accompanied by noticeable body vibration. This typically occurs because the engine needs to output extra power after the AC compressor engages. If the vehicle is older, worn components like the idle control valve or oxygen sensor can disrupt this balance. My old car had severe carbon buildup in the fuel injection system, causing uneven fuel delivery. I recommend monitoring RPM fluctuations when turning on the AC – if abnormal, inspect promptly to avoid roadside emergencies. Don’t underestimate long-term idle instability; it accelerates engine wear and increases fuel consumption.
