What Causes Unstable Idling When the Air Conditioner is On?
2 Answers
The causes of unstable idling when the air conditioner is on include engine carbon deposits, ignition system failure, and fuel pressure issues. Severe engine carbon deposits: Engine carbon deposits are the main cause of car shaking. When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold-start fuel injector is largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during startup. This can lead to unstable idling or even stalling. It is necessary to clean the fuel system, inspect the idle control valve, and clean the throttle body. Ignition system failure: The ignition system mainly consists of spark plugs and ignition coils. Prolonged use without replacing the spark plugs can cause the electrode gap to widen and carbon deposits to form on the electrodes, both of which can affect spark plug ignition and lead to unstable idling. If excessive carbon buildup or an overly large electrode gap is found on the spark plugs, they should be replaced. Fuel pressure issues: Whether the fuel supply pressure and intake pressure sensor are functioning properly is crucial. If the fuel pump supply pressure is abnormal or the intake pressure sensor provides incorrect readings or malfunctions, it can cause the car to shake or even stall. The fuel pump should be inspected and repaired or replaced if necessary.
The other day I was driving my old Ford when the weather got hot, so I turned on the AC. Suddenly, the idle started bouncing up and down like it was dancing—pretty scary. I asked a mechanic about it, and he said the compressor kicking in increases the engine load, but the control system can't keep up. Common causes include a dirty or clogged idle control valve affecting smooth adjustment, severe carbon buildup in the throttle body making the compressor load harder to handle, or aging sensors like the mass airflow sensor sending inaccurate data to the ECU, causing erratic idle. After cleaning the control valve, it went back to normal, but the mechanic suggested regular checks on these parts, especially in summer, to avoid breaking down and safety risks. Also, unstable voltage from the battery or alternator could be a factor, so preventive maintenance is best.
