What Causes Vehicle Shaking When Idling with AC On?
4 Answers
Here are the reasons for engine shaking when idling with AC on: 1. AC not increasing idle speed: Normally, a car's idle speed is maintained around 800 rpm. When AC is turned on, the idle speed should correspondingly increase to generally over 1000 rpm to prevent shaking. 2. Faulty ignition coil or spark plug: If the vehicle experiences slight hesitation during acceleration or minor idle instability without AC, but shows obvious idle shaking with AC on, it indicates issues with ignition coil, spark plug, or cylinder wires, suggesting incomplete combustion or cylinder misfire. 3. Excessive compressor pressure: Overly high AC pressure increases engine load and causes shaking. Common causes include poor condenser heat dissipation, excessive refrigerant, or dirty AC lines leading to high pressure, which may indirectly cause vibration.
I've been driving for over a decade and often encounter engine shaking when idling with the AC on, especially noticeable during hot weather at traffic lights. This mainly occurs because when the AC is turned on, the compressor activates, suddenly increasing the engine load. If the spark plugs are old with carbon buildup or the air-fuel ratio is incorrect, the idle becomes unstable and shaky. From my experience, the first step is to check the spark plugs and throttle body, cleaning them if necessary to reduce carbon deposits; worn engine mount bushings can also easily transmit vibrations and should be replaced early to prevent damage to other components. Additionally, pay attention to air filter cleanliness and regular oil changes, as these measures can prevent most issues. Never delay addressing these problems, as worsening vibrations can damage the engine, leading to higher repair costs and compromising driving safety.
My previous car was an older model SUV, and it was quite annoying when the body shook at idle with the AC on during summer. The repair shop diagnosed it as an electrical system issue. When the AC compressor consumes a lot of power and the battery or alternator can't supply enough, the engine RPM fluctuates, causing shaking. Worn engine mounts losing their damping effect is another common reason, as load changes transmit vibrations directly to the frame. Later, I replaced the battery and spark plugs, which improved the situation significantly. I recommend owners pay more attention to maintenance records, such as regular fuel injector cleaning and checking wire connections. Addressing issues promptly can save fuel and extend lifespan—don't wait until the shaking becomes too frequent to take action.
I often encounter the issue of car shaking when idling with the AC on during family trips, making everyone uncomfortable and affecting the mood. The mechanic explained that this happens because the AC compressor suddenly increases the engine load at startup, and since the idle speed is already low, any inefficiency in the ignition system or fuel supply can easily cause shaking. It's recommended to check the spark plugs and air filter during annual maintenance to ensure efficient operation. Older vehicles, in particular, require more attention to prevent such issues, ensuring a safer and more comfortable driving experience.
