What causes unstable idle speed when the car air conditioner is turned on?
3 Answers
Unstable idle speed when the car air conditioner is turned on is due to excessive engine carbon deposits or ignition system failure. Below are the related causes and solutions: 1. Severe engine carbon deposits: Engine carbon deposits are the main cause of car shaking. When there is too much carbon deposit inside the engine, the gasoline sprayed by the cold start injector will be largely absorbed by the carbon deposits, resulting in a lean mixture during startup, which can cause sudden stalling. Solution: Clean the fuel system, check the idle speed motor, and clean the throttle body. 2. Ignition system failure: Spark plugs are wear parts and need to be replaced regularly. Solution: Replace the spark plugs in a timely manner.
My car used to shake at idle when the AC was on, like the engine was gasping for air. Probably because turning on the AC suddenly increased the engine load—that compressor needs power to run, and the idle control system couldn't keep up. Common issues could be a dirty or stuck idle valve, a grimy throttle body affecting airflow, or worn spark plugs/ignition coils causing unstable power output. Sometimes it's related to the engine control unit too—if sensor signals go haywire, adjustments fail. This kind of problem tends to worsen over time. I later took it to a repair shop where they cleaned the throttle body and replaced the idle valve, which fixed it. Regular engine maintenance is key—clean carbon deposits and address issues promptly to avoid roadside breakdowns.
Having worked in auto repair for years, I've encountered many cases of unstable idle when the AC is turned on. The main reason is that compressor operation causes a sudden spike in engine load, and if the idle control system malfunctions, it leads to shaking. Key inspection points include whether the idle air control valve operates smoothly, if the compressor is seized or leaking refrigerant, whether the throttle position sensor signals are accurate, and checking vacuum lines for leaks. These issues often start with simple cleaning solutions, but don't attempt major repairs yourself – using a multimeter for circuit testing is fine, but major overhauls should be left to professional technicians. Additionally, engine carbon buildup and aging ignition systems can worsen the situation, so regular maintenance like spark plug replacement and intake tract cleaning is crucial for prevention, avoiding sudden engine stalls at high speeds.
