What Causes BMW Engine Vibration at Idle When AC is On?

[LaMatthew]

Here are the specific reasons for engine vibration at idle when the AC is on: 1. Shock absorber system failure: The shock absorber system is primarily responsible for filtering engine vibrations during operation. If there's an issue with the shock absorbers, these vibrations will directly transmit to the steering wheel and cabin, affecting driving. When the engine is idling, activating electrical systems can easily cause vibrations. The engine experiences increased load from electrical usage, causing idle speed to drop. To compensate for this vibration caused by idle speed fluctuation, the engine automatically increases RPM. When turning on electrical systems, especially the AC, engine vibration often occurs. This vibration is likely caused by aging shock absorbers and requires timely replacement. 2. Unstable oil pressure: If the engine has been decarbonized, the throttle body cleaned, and engine mounts replaced, but vibration persists, it's recommended to check fuel supply pressure and intake pressure sensors at a repair shop. Abnormal fuel pump pressure or malfunctioning intake pressure sensors can both cause vehicle vibration. 3. Cold start: The main reason for vibration during cold starts is incomplete fuel combustion, resulting in insufficient power. With lower ignition energy during startup when both fuel and lubricant temperatures are low, more throttle input is needed to meet power demands. Over time, spark plug electrode gaps gradually increase, leading to reduced ignition energy and poor fuel atomization, which causes vibration. Additionally, aging ignition coils, deteriorated spark plug wires, or electrical leakage can also reduce ignition energy.