What Causes Idle Shaking When AC is On?
3 Answers
Reasons for idle shaking when AC is on are as follows: Excessive Carbon Deposits: Severe carbon buildup in the engine cylinders can lead to clogged spark plugs and valves, directly causing the car to shake at idle. If the vehicle's acceleration performance has significantly declined and the shaking persists, an engine cylinder cleaning may be necessary. Fuel Supply System Malfunction: Unstable fuel pressure can cause inconsistent air-fuel mixture concentration, resulting not only in shaking but also unstable idle. Consider replacing the fuel filter first, as prolonged use may clog the filter element. Then check the fuel pressure bypassing the fuel filter; if abnormal, the issue may lie with the fuel pump. Engine Mount Deterioration: This is also a very common cause of shaking. If the engine shaking worsens over time but the power output does not weaken accordingly, it may be due to worn engine mounts.
When I first bought my car, I often encountered shaking at idle when the AC was on, which was quite annoying. The main reason is that once the AC compressor kicks in, the engine has to work extra to maintain operation, causing a sudden increase in load. If the idle control valve is dirty or the throttle body has carbon buildup, it can't adjust the RPM properly, making the car shake noticeably. Additionally, issues in the ignition system—such as worn spark plugs or unstable ignition coils—become more apparent under high load, leading to incomplete combustion of the air-fuel mixture. A loose or worn drive belt can also affect the compressor's performance, making the engine struggle and shake. The solution is to regularly inspect these components, clean carbon deposits, or replace old parts. I personally do an annual maintenance check, which has reduced a lot of issues. If the shaking gets severe, temporarily turning off the AC can help, but don’t delay long-term fixes to avoid engine damage and higher fuel consumption.
My car always shakes when idling with the AC on. I consulted friends who know about cars. They said that after the AC compressor starts, the engine bears a heavier load, and if the idle management doesn't keep up, the RPM will fluctuate. A common issue is spark plugs that have been used for too long and have surface erosion, causing unstable ignition under high load and leading to shaking. On the fuel supply side, clogged fuel injectors or insufficient fuel pressure from the fuel pump can starve the engine, making it prone to shaking. A vacuum system leak can also worsen the air-fuel ratio. They suggested checking simple things first, like the condition of the spark plugs—replace them with cheap ones if they're bad, don't wait until the shaking becomes frequent. Daily driving tips include warming up the car before turning on the AC to reduce impact. Anyway, just fix minor issues promptly.
