Reasons Why a BMW Shakes During Cold Start but Returns to Normal After a While

[Suzanne]

Analysis of the reasons why a BMW shakes during cold start but returns to normal after a while can be divided into two scenarios: 1. If the car runs normally when warm, it is likely caused by excessive carbon buildup in the intake manifold: Because gasoline atomization is poor during cold starts, the fuel injectors spray gasoline, some of which is absorbed by the carbon deposits. Only a portion of the gasoline enters the cylinders, resulting in a lean mixture (more air, less fuel). 2. When the car has been parked for a while, it may shake slightly during cold starts and when idling in neutral. The reasons are as follows: 1. Low ignition energy causing shaking: When the engine temperature is too low, both fuel and lubricant temperatures are low. During a cold start, a richer air-fuel mixture is required, which in turn demands higher ignition energy. Excessive spark plug gap, aging ignition coils, or high-voltage coils can all lead to reduced ignition energy, causing the car to shake. 2. Incorrect air-fuel mixture ratio causing shaking: During a cold start, if certain engine sensors or their signals have significant errors, the ECU may make incorrect judgments. This can result in inaccuracies in fuel injection timing and ignition timing control, leading to engine shaking. 3. Uneven cylinder conditions causing engine shaking: In multi-cylinder engines, significant differences in ignition energy between cylinders, as well as varying degrees of wear on pistons and piston rings, can cause inconsistent compression ratios. This leads to uneven power output from the engine, resulting in shaking.