What Causes High Engine Idle Speed?
2 Answers
Reasons for high engine idle speed: 1. Intake manifold or various valve leaks, when air, gasoline vapor, or combustion exhaust that should not enter the intake manifold causes the air-fuel mixture to be too rich or too lean, resulting in abnormal engine combustion; 2. Excessive carbon buildup on the throttle body and intake passages, preventing the control unit from accurately controlling idle air intake, leading to an overly rich or lean mixture and abnormal combustion; 3. Faulty idle air control components, causing inaccurate idle air control; 4. Valve train issues, where faults in the valve train result in excessive power loss in individual cylinders, leading to imbalance in power output among cylinders.
The engine idle speed may increase due to excessive carbon buildup in the throttle body, causing poor opening and closing, and the ECU compensates by raising the RPM; a faulty mass airflow sensor or idle control valve may send incorrect signals to the ECU, making the engine think it needs more power; if there is a vacuum hose leak, extra air entering disrupts the air-fuel mixture balance, naturally increasing the RPM; clogged or uneven fuel injector pressure may cause incomplete combustion, prompting the system to automatically raise the RPM for stability; a malfunctioning coolant temperature sensor can mislead the ECU into thinking the temperature is rising, activating a high-RPM protection mode. These are common causes. I recommend first checking the hoses and cleaning the throttle body. If the issue persists, visit a professional shop for an ECU diagnostic scan.
