Why does a BMW car have unstable idle speed when starting cold?
2 Answers
BMW car unstable idle speed when starting cold is mainly caused by excessive carbon deposits, throttle valve and air flow abnormalities. Excessive carbon deposits in the car: The reason for the unstable idle speed of the BMW N55 when cold is excessive carbon deposits in the throttle valve and air flow meter. Or there may be too much carbon deposits in the cylinder combustion chamber and pistons. Solution: Clean the carbon deposits in the car in time, and pay attention to the cleaning of engine carbon deposits and throttle valve during maintenance. Abnormal throttle valve and air flow: The reason for the unstable idle speed of the BMW N55 when cold is abnormal throttle valve and air flow. The throttle valve and air flow meter are prone to getting dirty. Most modern cars use electronic throttle valves. Solution: Inspect and repair the throttle valve and air flow in time.
I've been driving a BMW for almost ten years and have encountered unstable idle speed during cold starts quite a few times. The main reason is excessive carbon buildup in the throttle body. When the engine isn't fully warmed up in cold weather, carbon deposits block airflow, naturally causing unstable idle. Additionally, if the spark plugs are old, insufficient ignition energy makes cold starts even harder. Another possibility is a dirty mass airflow sensor, causing the ECU to read inaccurate data and resulting in fluctuating engine RPM. My car shows this most noticeably in winter, but it improved significantly after cleaning the throttle body. Occasionally, dirty fuel injectors can also cause trouble, especially in BMW's precision systems which have high fuel quality requirements – using low-grade fuel can exacerbate the issue. The checks are simple: regular cleaning during routine maintenance is sufficient. Prevention is always more cost-effective than repairs.
