What Causes High Idle Speed?
2 Answers
The causes of high idle speed in a car include a faulty idle control valve, air intake system leaks, fuel injector dripping or clogging. The reasons for sudden high idle speed are as follows: Faulty Idle Control Valve: The normal idle speed of an electronic fuel injection engine is determined by the idle control valve. The electronic control unit (ECU) calculates signals from engine speed, temperature, throttle position, and air conditioning switch, then adjusts the idle control valve to open the intake bypass channel or directly increase the throttle opening to raise the intake volume, thereby increasing the engine idle speed. If oil dirt or carbon deposits cause the idle control valve to stick or the throttle to fail to close properly, the ECU cannot accurately regulate the engine idle speed. Air Intake System Leaks: Normally, the intake volume must be strictly controlled. If there is a leak in the intake pipe, the air flow sensor cannot measure the actual intake volume, leading to inaccurate intake control by the ECU and resulting in unstable idle speed. Fuel Injector Dripping or Clogging: If the fuel injector drips or clogs, it cannot inject fuel according to the ECU's instructions, causing the air-fuel mixture to become too rich or too lean. This leads to poor performance in individual cylinders and unstable idle speed.
My car used to have high idle speed issues. After checking for a long time, I found the most common cause is a dirty throttle body, where carbon buildup jams it and causes inaccurate air intake. Another time, the idle speed control valve position sensor was faulty, preventing the ECU from controlling the RPM properly. If there's a vacuum hose leak, air entering the engine through cracks can also raise idle speed – testing leak points with cigarette smoke works amazingly well. Does idle speed surge when turning on AC or steering? That's normal, as the system automatically increases fuel to maintain power. Old cars are most vulnerable to coolant temperature sensor failures – if it falsely reports low temperature, the ECU will keep supplying extra fuel to raise RPM. High idle during cold startup is fine, but if it doesn't stabilize after warming up, it's time for inspection. I always ask mechanics to clean carbon deposits during maintenance, which prevents 80% of idle speed problems.
