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What is the reason for the occasional jerking during cold starts of the BMW 3 Series?

1 Answers

OAlexandra

07/30/25 9:01am

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.

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