What is the reason for the occasional jerking during cold starts of the BMW 3 Series?
3 Answers
BMW 3 Series may jerk during cold starts due to low-temperature starting and high engine RPM. Symptoms of jerking: When the car is started cold or at relatively low temperatures, symptoms such as engine shaking, high idle speed, transmission hesitation, and brake noise may occur. These are often accompanied by a strong odor of unburned gasoline and black smoke. Reasons for jerking: High RPM: It is normal for the idle speed to be higher during startup. The car requires higher RPM to allow the engine to reach normal operating conditions quickly, which necessitates a richer air-fuel mixture. As a result, both air intake and fuel injection are increased, leading to higher RPM. The colder the weather and the lower the water temperature (compared to the normal engine operating temperature, typically around 85 to 110 degrees Celsius), the greater the temperature difference from normal engine operation. The car's computer will control a higher cold-start idle speed, meaning the engine RPM will be higher when first started. Under normal circumstances, depending on the water temperature, the engine's cold idle speed will gradually decrease from over 1200 RPM until the engine reaches its normal operating temperature and stabilizes at the standard idle speed of around 800 RPM. The solution is to clean the throttle body promptly, which generally resolves the issue.
My BMW 3 Series occasionally jerks during cold starts, which is quite annoying. Common causes include ignition system failures. Worn spark plugs or loose ignition coils can lead to insufficient spark during cold starts, resulting in unstable engine combustion. It could also be due to poor fuel injection, where carbon buildup in the fuel injectors causes uneven fuel delivery, leading to jerking. I've also seen cases where faulty intake sensors mislead the ECU into adjusting the air-fuel ratio incorrectly. Regular maintenance is crucial—check spark plugs every 20,000 km and clean the throttle body periodically to prevent carbon buildup. Using higher-octane gasoline reduces deposit formation, effectively preventing this jerking issue. Ignoring it can worsen engine wear over time, increase fuel consumption, and even affect emissions. Addressing it early saves hassle, money, and ensures safety.
As a long-time BMW owner, I have personally experienced the jerking during cold starts. The issue may lie in the idle control system, such as carbon buildup in the idle air control valve or a faulty throttle position sensor, causing unstable fuel supply to the engine. Fuel system problems like insufficient fuel pump pressure or clogged filters can also affect power output when cold. Additionally, BMW's electronic throttle is temperature-sensitive and particularly prone to issues in winter. I recommend first reading the trouble codes to identify the root cause, rather than randomly replacing parts and wasting money. For daily maintenance, pay attention to regular oil changes and air filter replacements. Keeping the intake system clean can prevent many minor issues, extend the vehicle's lifespan, and reduce the risk of unexpected stalling.
