Why does the car's idle speed fluctuate when the air conditioning is on?
2 Answers
The reasons for unstable idle speed fluctuation when the car's air conditioning is on are as follows: 1. Engine control logic error, unable to receive the air conditioning activation signal: When unable to determine whether the air conditioning is on, the engine control unit (ECU) mistakenly perceives it as a temporary high load and attempts to increase the speed to counteract the idle resistance. When the engine speed decreases, it cannot handle the load of the air conditioning compressor and raises the speed again. This back-and-forth results in unstable idle speed; 2. Air conditioning control circuit error, unable to send the air conditioning activation signal: Similar to the first point, because the air conditioning status cannot be determined, the ECU continuously adjusts the idle speed, leading to unstable idle; 3. Air conditioning circuit malfunction, causing intermittent operation of the air conditioning: When the air conditioning is not working, the air conditioning compressor belt runs without load, so it does not affect the engine load. The intermittent operation of the air conditioning compressor causes the engine to continuously adjust the idle speed, resulting in unstable idle.
This situation is quite common. From my driving experience, it usually happens when the air conditioning is turned on, as the compressor starts and increases the engine load. When the car engine is idling, the computer automatically adjusts the RPM to maintain stability. However, once the AC is turned on, the sudden demand for power from the compressor can easily cause idle fluctuations, like uneven shaking or surging. Common causes may include excessive carbon buildup in the throttle body, a faulty or dirty idle control valve—these issues can occur in both old and new cars. I've driven many models, and some older cars are more prone to this problem due to long-term carbon deposits making the throttle less responsive. Newer cars might also experience it if sensors send incorrect signals to the computer. I recommend first checking if the throttle body is clean—cleaning it yourself might help if it's easily accessible. In severe cases, you should visit a repair shop to inspect the idle system or AC components. Regular maintenance, like periodic engine cleaning, can prevent excessive engine strain when using the AC.
