What Causes Engine Idle Vibration When the Car Air Conditioner is On?
2 Answers
Unstable idle speed when the car air conditioner is on is generally caused by excessive engine carbon deposits. This occurs because the air conditioner compressor increases the engine load when activated, while the engine controls the throttle opening and fuel injector quantity based on standard parameters. Additional relevant information is as follows: Note 1: If carbon deposits accumulate on the throttle body and fuel injectors, they will affect the actual air intake and fuel injection volume. Carbon deposits also impair the atomization effect of the air-fuel mixture and the air-fuel ratio. Note 2: When the engine lacks sufficient idle power to handle the load from the air conditioner compressor, the idle speed will gradually decrease. As it approaches the minimum threshold, the engine will repeatedly adjust the air intake and fuel injection, resulting in the observable phenomenon of unstable idle speed.
When driving with the AC on, the car shakes at idle. I often encounter this issue, especially when starting the cold air on hot summer days. The main reason is that when the AC compressor kicks in, the engine load suddenly increases. At idle, the engine is already running unstably, and the added load makes the vibration worse. This is common in older cars. For example, my first car shook so badly that I had to get it repaired, and it turned out the engine mounts had aged. The mounts are rubber vibration dampeners that can crack or harden over time, failing to stabilize the engine. Additionally, if the spark plugs or ignition coils are worn, the ignition efficiency drops, making it harder to maintain balance when the compressor starts. Carbon buildup in the fuel injectors or a dirty idle control valve can also cause issues. Prolonged shaking isn’t just uncomfortable—it can accelerate component wear and increase fuel consumption. I recommend first checking the condition of the mounts, then cleaning the throttle body and inspecting the ignition system. If necessary, replace the spark plugs or use a fuel additive to clean the internal system. Regular engine maintenance is crucial, especially during seasons when the AC is used frequently, to ensure the fuel and electrical systems operate smoothly.
