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What are the reasons for unstable idle speed of BMW N55 when cold?

2 Answers

DiEva

07/30/25 2:09am

There are two main reasons for the unstable idle speed of BMW N55 when cold. Excessive carbon deposits in the car: The reason for the unstable idle speed of 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. It is necessary to clean the carbon deposits in the car in time, and pay attention to the cleaning of engine carbon deposits and throttle valve during maintenance. Slight carbon deposits are not a problem and can even be naturally eliminated during driving. However, severe carbon deposits can change the original structural parameters of the engine, reduce engine power, and increase fuel consumption; in severe cases, it may also cause engine knocking, acceleration noise, damage to pistons and crankshafts, and even lead to the engine burning oil and requiring major repairs. Abnormal throttle valve and air flow: The throttle valve and air flow should be checked and repaired in time.

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KingstonRose

08/15/25 8:10am

I've seen quite a few cases of unstable idle speed in BMW N55 engines when cold at repair shops, which is a fairly common issue. It's usually caused by faulty sensors, such as skewed readings from the mass airflow sensor or aging oxygen sensors. When the engine is cold and oil temperature is low, inaccurate sensor signals can lead the ECU to miscalculate the air-fuel mixture ratio. Another common cause is clogged or carbon-fouled fuel injectors leading to unstable fuel supply, or worn-out spark plugs failing to ignite properly—especially common with leaking ignition coils. Additionally, vacuum leaks can cause uneven air intake, making the idle shake as if the engine is about to stall. I recommend getting it checked as soon as possible. Use a diagnostic tool to read the trouble codes and troubleshoot step by step. Don't ignore this issue, as sudden shaking while driving can be very dangerous. I've fixed several vehicles by cleaning the throttle body or replacing ignition components.

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