What Causes a BMW to Shake and Stall After Releasing the Brake Upon Starting?
2 Answers
The reason a BMW shakes and stalls after releasing the brake upon starting is due to a dirty throttle body or excessive carbon buildup in the fuel injectors. Introduction to the Throttle Body: The throttle body is the gate that controls the flow of air into the engine. When external air enters the engine, it mixes with gasoline to form a combustible mixture. The upper end of the throttle body connects to the air filter, while the lower end connects to the main engine body. When the combustible mixture is formed, it enters the engine through the intake manifold, triggering the engine's power system. The driver controls the throttle opening by operating the accelerator pedal. Introduction to the Fuel Injectors: The fuel injector is a simple solenoid valve. When the electromagnetic coil is energized, it generates a magnetic force that lifts the needle valve, opening the injection nozzle. Fuel is sprayed at high speed through the annular gap between the needle valve tip and the nozzle, forming a fine mist for optimal combustion.
I've been driving BMWs for many years, and this situation is usually caused by an issue with the idle control system. When releasing the brake, the engine maintains idle at low RPM. If the idle motor or throttle position sensor is faulty, the ECU can't stabilize the speed, causing the car to shake and then stall. It could also be due to worn spark plugs or faulty ignition coils, leading to misfires in individual cylinders and overall instability. BMW's automatic transmission is linked to the brake system, so abnormal brake switch signals might interfere with idle control. I remember helping a friend fix his car last year - it was just excessive carbon buildup in the throttle body, and cleaning solved the problem. Check if the air filter is dirty, as it can affect air intake. For safety, don't force the car to drive and take it to a professional shop immediately for diagnosis to avoid worsening the problem and causing bigger failures.
