What Causes Unstable Engine Idling Speed?

[VonHadley]

The reasons for unstable engine idling speed are as follows: 1. Intake manifold or various valve leaks: When unwanted air, gasoline vapor, or combustion exhaust enters the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to abnormal engine combustion. 2. Excessive carbon buildup in the throttle body and intake passages: Common phenomena include oil deposits or carbon buildup on the throttle body; oil deposits or carbon buildup in the intake passages around the throttle body; oil deposits or carbon buildup on the idle speed stepper motor, duty cycle solenoid valve, and rotary solenoid valve. 3. Faulty idle air control components: Malfunctioning idle air control components result in inaccurate idle air control. Common causes include damaged or stuck throttle motor; damaged or stuck idle speed stepper motor, duty cycle solenoid valve, and rotary solenoid valve.