What Causes Engine Shaking at Idle When the AC is On?
2 Answers
Here are the reasons for engine shaking at idle when the AC is on: 1. No RPM increase with AC activation: Typically, a car's idle speed is maintained around 800 RPM. When the AC is turned on, the idle speed should normally increase accordingly, usually staying above 1000 RPM to prevent idle shaking. 2. Faulty ignition coil or spark plug breakdown: If the vehicle experiences slight hesitation during acceleration or minor idle instability without AC operation, but shows pronounced idle shaking when the AC is activated, this indicates issues with the ignition coil, spark plugs, or cylinder wires. Possible causes include cylinder misfire or incomplete combustion. 3. Excessive compressor pressure: Overly high AC system pressure increases engine load and may cause shaking. Common culprits include poor condenser heat dissipation, excessive refrigerant, or dirty AC lines - all potentially leading to excessive pressure and indirect vibration.
Once when I was driving with the AC on, I noticed the idle was shaking badly, making the whole car vibrate, which was quite annoying. This is usually related to the increased load on the engine, as the AC compressor draws power when activated, making it harder to maintain a stable idle speed. There could be many reasons: aging of the engine's ignition system, like carbon buildup on the spark plugs; or clogged fuel injectors leading to insufficient air-fuel mixture; a dirty idle control valve or throttle body that can't respond quickly to load changes. Driving like this for a long time can increase fuel consumption and may be accompanied by strange noises or lack of power. Remember to perform regular maintenance, check the spark plugs and air filter, and don't wait until the shaking gets severe to fix it—sudden engine stalls on the road can be dangerous, especially in summer traffic jams. My personal habit is to clean the throttle body regularly, which can reduce the chances of shaking.
