What Causes Unstable Idle and Shaking in a Warm Car?

[LaneAnn]

Unstable idle and shaking in a warm car are commonly caused by a dirty throttle body or throttle body malfunction. Causes of Shaking: Severe engine carbon buildup is the most frequent reason for car shaking, often due to an excessively dirty throttle body or excessive carbon deposits in the fuel injectors. When there's too much carbon buildup inside the engine, the gasoline sprayed by the cold start injector can be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts. Ignition system issues—check the condition of the spark plugs, high-voltage wires, and ignition coils. Poor performance in the ignition system or inadequate spark plug firing can also lead to such symptoms. Solutions: Clean the fuel system and check if the idle speed control valve has carbon buildup—it should be cleaned if necessary. Inspect the spark plugs for excessive carbon deposits and replace them if needed. Check the fuel pressure and replace components as required.