What Causes BMW Engine Shaking at Idle?

[VanGabriela]

BMW engine shaking at idle is generally caused by severe carbon buildup in the engine, issues with the ignition system, or unstable fuel pressure. The specific reasons are analyzed as follows: 1. Severe engine carbon buildup: When there's excessive carbon deposit inside the engine, the gasoline injected by the cold-start fuel injector will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts and making ignition difficult. In this situation, the engine only starts easily after the carbon deposits become saturated with gasoline. After starting, the gasoline adsorbed on the carbon deposits will be drawn into the combustion chamber by the engine's vacuum suction and burned, making the air-fuel mixture too rich. This alternation between lean and rich mixtures causes idle shaking after cold starts. The lower the temperature, the more fuel is required for cold starts, and carbon deposits will significantly affect whether cold starts proceed smoothly. 2. Ignition system problems: Poor spark plug performance can also lead to such shaking phenomena. 3. Unstable fuel pressure: Abnormal fuel pump pressure or malfunctioning intake pressure sensors can both cause vehicle shaking.