What Causes Engine Shaking at Idle with AC On but Not When Accelerating?
4 Answers
The reasons for engine shaking at idle with AC on but not when accelerating: Ignition system issues: Spark plugs have a limited lifespan. If spark plugs have been used for an extended period, they may exhibit poor sparking performance, leading to unstable idle conditions. Therefore, it's essential to promptly inspect the ignition system, including checking whether the spark plugs need replacement or if there's excessive carbon buildup. If such issues are confirmed to be causing idle shaking, immediate replacement of spark plugs or cleaning is necessary. Throttle body requires cleaning: The proper functioning of the throttle body directly affects the engine's operational stability. During vehicle operation, carbon deposits can accumulate in the throttle body. Excessive carbon buildup can obstruct normal engine air intake, resulting in insufficient engine power to adequately drive the compressor or barely managing to do so. In such cases, activating the AC at idle may cause engine shaking. Thus, it's crucial to inspect the throttle body for cleanliness and perform timely cleaning if necessary.
I've also experienced this situation. When the car is idling with the AC on, the steering wheel shakes noticeably, but as soon as I press the accelerator to speed up, the shaking disappears, which is quite alarming. The reason is that when the AC is on, the compressor activates, adding extra load to the engine, especially at idle when the engine speed is low and power is insufficient, making it prone to shaking. It could also be due to loose or worn engine mounts, causing vibrations to transfer to the body, or excessive carbon buildup on the spark plugs and ignition system issues, preventing smooth operation. Additionally, a dirty throttle body causing poor airflow can lead to this. I recommend checking the condition of the engine mounts and cleaning the throttle body, as engine load issues can easily arise, and neglecting them long-term might damage other components.
My car used to shake badly when idling with the AC on, even the seats were vibrating, but it ran smoothly when driving. I asked a mechanic, who said this is quite common because the AC compressor requires more engine power when working. At idle speed, the engine RPM is low, and the increased load causes shaking; when you step on the gas and the RPM rises, the engine handles it easily and stops shaking. The key issues might be aging ignition coils, worn-out spark plugs needing replacement, or insufficient fuel pump pressure causing unstable fuel supply. Later, I tried cleaning the throttle body and replacing the spark plugs myself, and the problem was solved. It's also important to check the engine mounts, as their shock absorption effect deteriorates when they age.
As a car enthusiast, I've analyzed this type of vibration issue. The car shakes when idling with AC on but remains stable during acceleration. This is mainly because the compressor increases engine resistance, consuming all available power at idle speed which isn't enough for smooth operation. During acceleration, the higher RPM provides sufficient horsepower to maintain stability. The root causes might be throttle carbon buildup restricting air intake, or deteriorated engine mount bushings failing to absorb vibrations properly. Additionally, compressor malfunctions can also cause uneven load distribution. Regular minor maintenance like throttle cleaning and mount inspections can effectively prevent this. The problem occurs more frequently during summer when AC usage increases.
