What Causes BMW Engine Idle Vibration When AC is On?
2 Answers
BMW engine idle vibration when AC is on can be caused by the following reasons: Severe engine carbon buildup: Engine carbon deposits are the main cause of car vibration. When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold start injector will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts, making cold starts more difficult. Ignition system malfunction: If the spark plugs have excessive carbon buildup or the ignition gap is too large, they need to be replaced. The working condition of high-voltage wires and ignition coils can be determined using the vehicle's computer to assess the ignition system's status. Fuel pressure issues: Whether the fuel supply pressure and intake pressure sensors are functioning normally. Abnormal fuel pump supply pressure or incorrect values and poor performance of the intake pressure sensor can all cause body vibration.
Last time I drove a BMW, the engine idled roughly as soon as I turned on the AC, which really worried me. After checking, I found out that the AC compressor increases the engine load when activated. If there's too much carbon buildup in a BMW engine under high load, it can block the intake manifold, causing uneven air intake and resulting in rough idling. Worn-out spark plugs are another reason—weak ignition leads to instability. It could also be due to a stuck or faulty idle control valve that fails to adjust the RPM promptly. Additionally, as the car ages, dirty fuel lines or filters can cause insufficient fuel supply. This kind of vibration isn't just annoying—it also affects driving safety. It's best to get it checked early at a repair shop for throttle body cleaning, before the problem worsens.
