Reasons for engine shaking during cold starts: 1. Severe engine carbon buildup: The most common cause of car shaking is excessive carbon deposits on the throttle valve or fuel injectors. When there's too much carbon buildup inside the engine, the gasoline sprayed by the cold-start injector gets largely absorbed by these deposits, resulting in an overly lean air-fuel mixture during cold starts. This makes ignition difficult - the engine only starts easily after the carbon deposits become saturated with gasoline. Once started, the gasoline adsorbed on the carbon deposits gets drawn into the combustion chamber by the engine's vacuum, making the mixture too rich. This alternating lean-rich mixture condition causes idle shaking after cold starts. Lower temperatures require more fuel for cold starts, making carbon deposits more likely to affect successful starting. 2. Ignition system issues: Check the condition of spark plugs, high-voltage wires, and ignition coils. Poor ignition system performance or weak spark plug firing can also cause severe shaking during cold starts. 3. Unstable fuel pressure: If you've already cleaned engine carbon deposits, flushed the throttle valve, replaced gaskets and spark plugs but still experience body shaking at idle, it's recommended to visit a 4S shop to check fuel supply pressure and the intake pressure sensor. Abnormal fuel pump pressure or faulty intake pressure sensor readings can both cause body shaking. 4. Aging engine components: Car shaking can also be related to worn engine mounts (also called rubber mounts). Engine mounts act as the engine's vibration absorption system, responsible for dampening minor vibrations during operation. If engine mounts fail, these vibrations get transmitted to the steering wheel and cabin, causing shaking at idle.
