What Causes Engine Idle Shaking When the AC is On?
2 Answers
Idle instability and shaking only occur when the air conditioning is turned on, and the possible causes are as follows: 1. Engine control logic error, failing to receive the AC activation signal. When unable to determine whether the AC is on, the engine control unit (ECU) mistakenly interprets it as a temporary high load and attempts to compensate by increasing the RPM to offset the idle resistance. When the engine speed drops, it cannot handle the load from the AC compressor, causing the RPM to rise again. This back-and-forth adjustment manifests as unstable idling. 2. AC control circuit error, failing to send the AC activation signal. Similar to the first point, since the ECU cannot determine the AC's status, it continuously adjusts the idle speed, resulting in unstable idling. 3. AC circuit malfunction, causing intermittent operation of the AC. When the AC is off, the compressor belt runs without load, so it does not affect the engine load. However, intermittent operation of the AC compressor forces the engine to repeatedly adjust idle speed, leading to unstable idling.
I've been driving for nearly twenty years, and I always feel severe shaking when idling with the AC on in summer. I remember during my last maintenance visit, the mechanic told me this is mainly because once the AC compressor kicks in, the engine load increases significantly, and the idle speed can't keep up, causing the shaking. He said the most common reasons are excessive carbon buildup on the throttle body, blocking the air intake, and aging spark plugs leading to uneven ignition. Later, I had the throttle body cleaned for about a hundred bucks and replaced the spark plugs, which solved the problem. Additionally, unstable battery voltage or a loose belt can also worsen the shaking. He suggested cleaning the throttle body once a year and checking these components in advance to save on major repair costs. Long-term neglect could lead to premature engine wear and significantly increased fuel consumption, which is not worth it.
