What Causes Fluctuating Idle Speed?

[McJulia]

Reasons for unstable engine idle speed: 1. Intake manifold leakage - If there's an intake manifold leak, it disrupts the gas balance inside the engine, causing either excessive or insufficient pressure, leading to unstable idle speed; 2. Carbon buildup - Throttle valve carbon deposits are the primary cause of fluctuating idle speed; 3. Ignition system failure - Issues like misaligned spark plugs, blown fuses, or faulty ignition modules can all cause unstable engine idle; 4. Poor fuel quality. Idling refers to an engine's operating condition when in neutral gear. The rotational speed during idling is called idle speed, which can be adjusted by modifying the throttle opening size.