What Causes the Engine to Shake and Stall During Cold Start?

[SabrinaRose]

The reason for the engine shaking and stalling during cold start is carbon buildup. Carbon deposits typically accumulate on the inner walls of the throttle body, the intake valve heads, the inner walls of the intake manifold, and the combustion chamber walls. Excessive carbon buildup on the throttle body's inner walls can restrict airflow, leading to difficulty starting or stalling at idle. Once the coolant reaches the engine's normal operating temperature, the throttle opening may exceed the normal idle value, causing the ECU to repeatedly adjust the throttle plate position, resulting in fluctuating idle speeds. If carbon deposits accumulate excessively on the intake valve heads, the valves may not seal properly, making cold starts even more difficult. Even if the engine manages to start, it may stall quickly. Additionally, as the engine warms up, the fuel absorbed by the carbon deposits evaporates and is drawn into the cylinders for combustion. This uncontrolled fuel supply can lead to an overly rich air-fuel mixture, further causing poor engine performance.