What Causes Severe Engine Shaking During Cold Start?

[DelHaley]

Severe engine shaking during cold start can be attributed to several factors: Poor combustion within the cylinders leads to insufficient power output, resulting in vehicle vibration. When the engine temperature is too low, both fuel and lubricating oil fail to reach optimal operating temperatures, requiring additional fuel injection during cold starts to meet power demands. Larger spark plug gaps reduce ignition energy - a critical issue during cold conditions when fuel atomization is poor and higher ignition energy is needed. Prolonged use causes spark plug gaps to widen naturally, diminishing ignition energy and consequently affecting engine performance, manifesting as shaking. Additionally, aging ignition coils, deteriorated spark plug high-voltage wires, or electrical leakage can similarly reduce ignition energy.

[DelLillian]

I find that cold start shaking is too common, mainly in three directions: First, there is a fuel system issue. At low temperatures, gasoline atomization is poor. If the fuel injectors are clogged with carbon deposits, causing uneven fuel injection, the air-fuel mixture concentration will be incorrect, making the engine shake. Second, there is the ignition system. If the spark plug gap is too large or the electrodes are worn, it's easier to have unstable ignition on cold winter mornings due to heavy moisture. If one or two cylinders don't work properly, the car will shake. Lastly, there are carbon deposits, especially on the back of the intake valves, which act like a thick cotton coat for the engine, obstructing air intake and disrupting the air-fuel mixture ratio. There's also a hidden boss—the engine mounts. When the rubber hardens due to aging, the damping effect is reduced, amplifying even minor vibrations and transmitting them into the cabin. It's recommended to start troubleshooting with inexpensive spark plugs and fuel additives first.

[StEmersyn]

Last week, my neighbor's car had the same issue—shaking like a sieve during cold starts. When I checked it for him, I focused on the sensors. Data drift from the crankshaft position sensor or the mass airflow sensor can cause the ECU to miscalculate the fuel injection volume. For instance, distorted intake air data can lead to overly lean fuel injection, resulting in incomplete combustion and shaking. Also, pay attention to the coolant temperature sensor. If it reports errors, the ECU may misjudge the temperature, injecting fuel based on hot-engine data during cold conditions, which is definitely problematic. For older cars, it's also important to monitor cylinder pressure. Wear inside the cylinders can reduce sealing efficiency, especially when the oil's fluidity is poor at low temperatures, leading to inadequate lubrication and uneven cylinder pressure, which can also cause shaking. For this, you’ll need a cylinder compression tester to get accurate data.

[MacAvery]

From a mechanical perspective, increased piston ring clearance is a hidden killer. After overnight parking, engine oil drains back to the oil pan, causing intensified metal-on-metal dry friction during cold starts and unstable power output due to insufficient cylinder pressure. Aging valve stem seals are equally troublesome, as oil seeping into the combustion chamber forms carbon deposits that compromise sealing. Modern direct injection engines are more prone to fuel injector carbon buildup, particularly when the injector nozzles near intake valves get clogged, altering spray patterns. Regular use of fuel injector cleaner is recommended, along with spark plug gap inspection every 20,000 km. Using 95-octane gasoline instead of 92-octane for better anti-knock performance is also worth trying.