What are the reasons for the shaking during cold start of BMW 5 Series?

[WalterLee]

The reasons for the shaking during cold start of BMW 5 Series are: 1. Excessive carbon deposits inside the engine, leading to an overly lean air-fuel mixture during cold start, causing shaking; 2. Mechanical failures such as ignition coils, spark plugs, or fuel injectors; 3. Low oil pressure or cylinder pressure, resulting in insufficient power; 4. Poor fuel quality; 5. Damaged or excessively dirty idle control valve; 6. Intake manifold leakage; 7. Damaged engine mounts or rubber pads; 8. Oxidized or low-grade engine oil. Taking the 2021 BMW 525li as an example, it is a mid-to-large-sized 4-door 5-seater sedan with a body size of 5106mm in length, 1868mm in width, and 1500mm in height, with a wheelbase of 3105mm. This car is equipped with a 2.0T turbocharged engine and an 8-speed automatic transmission, delivering a maximum power of 135 kW and a maximum torque of 290 N·m.

[LeFinn]

Having driven a BMW 5 Series for seven or eight years, I've noticed that cold start shaking is quite common. The most common issue is carbon buildup, especially on the intake valves and throttle body. During cold starts, gasoline atomization is poor, and carbon deposits absorb fuel, leading to incorrect air-fuel mixture ratios. Another cause is aging spark plugs; after 30,000 to 40,000 kilometers, their ignition energy decreases, and the higher compression ratio during cold starts can cause shaking. Poor fuel quality can also have an impact, as clogged fuel injectors disrupt smooth fuel supply. A reminder to everyone: in winter, avoid turning on the heater immediately—give the engine a few dozen seconds to warm up. Regularly adding fuel system cleaner and driving at high speeds to clear carbon deposits is quite effective. If the shaking is severe, it's best to visit a shop for a computer diagnosis to check for sensor faults.

[KeeganDella]

In repair shops, BMW 5 Series cars with cold start shaking are frequently seen. The ignition system is the most critical. If the spark plug gap is too large or the ignition coil is leaking, the insufficient high voltage when cold causes cylinder misfire. Excessive deposits on fuel injectors leading to poor atomization is also a main reason, as the fuel injection volume during cold starts is about 20% higher than usual. Incorrect oil viscosity with poor fluidity in winter results in inadequate lubrication and increased friction. It is recommended to check the engine mount rubber pads, as aged ones have poor damping effects, making the shaking more noticeable. The BMW B48 engine is particularly prone to carbon buildup, so it's necessary to remove and clean the throttle body every 20,000 kilometers—don't wait until the fault light comes on to address it.

[StAniyah]

Research on engines reveals that cold engine shaking is essentially caused by unstable combustion. Inaccurate water temperature sensors in BMWs can lead the ECU to misjudge and inject less fuel, resulting in a lean mixture and causing shaking. Intake manifold leaks allow extra air to enter, disrupting the air-fuel ratio. Another potential issue is the VVT solenoid valve sticking during startup, affecting valve timing. It's recommended to check the carbon canister solenoid valve, as it can cause an overly rich mixture if stuck open. BMW engines warm up quickly, but in northern sub-zero conditions, it's better to wait until the RPM drops to 800 before starting off, which also benefits transmission protection.