Why does the car vibrate when idling with the AC on?

[MacGianna]

When idling with the AC on, the car may vibrate due to the following reasons: Carbon buildup in the car: As the vehicle accumulates more mileage, carbon deposits can form in various parts, especially in the throttle body. If the throttle body is excessively dirty, it can obstruct air intake, causing the engine to struggle or barely manage to power the compressor. This results in noticeable car vibrations when the AC is turned on. Engine mount failure: The engine naturally vibrates during operation (including when the compressor is activated), but these vibrations are usually dampened by the engine mounts, making them less noticeable to the driver. However, if the engine mounts are collapsed or damaged, the engine's balance is disrupted. When the compressor is engaged, it may reach a resonant frequency, intensifying the vibrations.

[TheoAnn]

I've always thought that resonance when the AC is on at idle is quite common, probably because my car is a bit old. Worn engine mounts are the most common cause. When the AC compressor kicks in, the engine load increases, but at low idle speeds, if the mounts aren't sturdy enough, the vibrations spread. Belt issues can also cause this, like looseness or aging, leading to squeaking and vibrations when the AC is running. Additionally, carbon buildup can make the engine idle unevenly, running poorly at low RPMs, and the added load from the AC makes resonance more likely. I always advise friends to first check the engine mounts for cracks or sagging, then inspect the belt tension, and clean the throttle body carbon buildup if necessary. Regular maintenance can reduce these issues—don't wait until a major repair is needed to regret it.

[SimonMarie]

I remember my first car often shook when idling with the AC on, which was quite annoying. I think it mainly had to do with the car's age—the engine mounts were worn out and couldn't absorb the vibrations. The AC compressor requires extra engine power to start, and since idle speed is already low, the vibrations naturally get amplified. It's even worse if the belt shows signs of slack. It could also be due to a dirty or slow-responding idle control valve, or an unbalanced cooling fan causing noise. Once, while fixing it myself, I found the mounts were broken, and replacing them solved the issue. For daily use, avoid running the AC at idle for too long, as it puts a heavy load on the engine and can cause overheating. It's safer to take it to a repair shop for a full diagnosis.

[MacMorgan]

Idle AC vibration is usually not complicated. Worn engine mounts are the main cause, as the compressor increases engine load causing shaking. Loose belt systems also contribute. Carbon buildup affecting idle stability is equally common. First, check engine mounts yourself: push the engine to see if it wobbles. Test belt tension with tools. Try cleaning carbon deposits. If unresolved, visit a professional shop – simple and quick.