What Causes BMW to Shake and Stall During Cold Start?
2 Answers
For BMW 5 Series shaking and stalling during cold starts, the most common cause is carbon buildup. Carbon deposits typically accumulate on the inner walls 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 performance. Below is a brief introduction to automotive carbon deposits: (1) Excessive carbon buildup on the inner walls of the throttle body can restrict airflow, leading to difficulty starting or stalling at idle. Once the engine reaches normal operating temperature, the throttle opening may exceed the normal idle value, causing the ECU to repeatedly adjust the throttle valve position, resulting in erratic idle speeds. (2) Carbon deposits have a porous, sponge-like surface. After the car has been parked overnight or for an extended period, during a cold start, most of the fuel injected by the injectors is initially absorbed by the carbon deposits, leaving insufficient fuel in the combustion chamber for proper ignition. Only after multiple injections during cranking does enough fuel enter the combustion chamber for ignition. This condition inevitably makes starting difficult. More severely, if carbon deposits accumulate excessively on the intake valve heads, causing the valves to seal improperly, cold starts become even more challenging.
As a BMW owner, I find it quite annoying when the car experiences shaking and stalling during cold starts. Common causes include unstable ignition due to aging spark plugs or carbon buildup, especially noticeable in low winter temperatures. The fuel system should not be overlooked either—for example, clogged fuel injectors can disrupt the air-fuel mixture ratio, and insufficient fuel pump pressure can make fuel delivery difficult. In the intake system, a dirty mass airflow sensor can affect data transmission to the ECU, leading to inaccurate control. Additionally, worn ignition coils or severe throttle body carbon buildup can cause unstable engine RPMs and subsequent stalling. During my last inspection, I used an OBD scanner to read the fault codes and discovered a sensor issue—replacing the part and cleaning the carbon buildup improved the situation. For prevention, it's advisable to use high-quality gasoline and additives regularly, and not to delay until the problem worsens.
