What are the reasons for BMW N54 cold start shaking?

[TimothyFitz]

For BMW 5 Series cold start shaking and stalling, the most common cause is carbon buildup. Carbon deposits typically occur in several areas: the inner wall of the throttle body, the intake valve heads, the inner walls of the intake manifold, the piston tops, and the inner walls of the combustion chamber. It is recommended that owners visit a repair shop to identify the specific cause and resolve it promptly to avoid affecting vehicle operation. Below is a brief introduction to automotive carbon deposits: (1) Excessive carbon buildup on the inner wall of the throttle body can lead to insufficient air intake, making starting difficult or causing the engine to stall easily at idle. After the coolant reaches the engine's normal operating temperature, because the throttle opening exceeds the normal idle value, the ECU repeatedly adjusts the throttle valve plate position, resulting in fluctuating idle speeds. (2) Carbon deposits have a relatively loose, sponge-like surface. After sitting overnight or for an extended period, during a cold start, most of the fuel injected by the fuel injectors is initially absorbed by the carbon deposits, leading to insufficient fuel entering the cylinders and a lean mixture that is difficult to ignite. As the starter operates and multiple injections occur, enough fuel enters the cylinders for ignition. In such cases, starting is inevitably difficult. More seriously, if carbon deposits accumulate excessively on the intake valve heads, causing the valves to not seal properly, cold starts become even more challenging.