The main reasons for high car idle speed are: 1. Excessive carbon buildup in the throttle body, which reduces engine air intake, causing the ECU to increase throttle opening to raise RPM; 2. Air leaks at the rear of the throttle body, allowing outside air to enter the engine and increase RPM; 3. Generator failure causing the ECU to increase RPM; 4. Damage to the throttle body itself or the idle speed control valve. Specifically, the reasons for high car idle speed include: 1. Excessive carbon buildup in the throttle body. This carbon buildup forms over time from the intake system as air contains impurities like dust. When carbon buildup is significant, the throttle's opening and closing resistance increases, and engine air intake decreases. Reduced air intake can cause stalling, so the ECU increases air intake and adjusts the throttle opening to prevent stalling, naturally raising the idle speed. 2. Air leaks at the rear of the throttle body, usually caused by aging or cracked intake manifold gaskets or damaged vacuum hoses. Since the rear of the throttle body is in a negative pressure vacuum zone, leaks allow outside air to be drawn into the engine. When the ECU detects increased air intake, it increases fuel injection, raising idle speed. 3. Generator failure. When engine RPM remains constant, the generator load increases with additional electrical devices. If the ECU detects the generator is not producing electricity, it assumes the engine RPM is too low and commands an increase in idle speed. 4. Damage to the throttle body or idle speed control valve. A faulty potentiometer inside the throttle body affects the throttle opening size, and a larger opening leads to higher idle speed. The idle speed control valve regulates idle speed, and if it fails, idle speed will also increase. Correspondingly, solutions for high car idle speed include: 1. For excessive carbon buildup in the throttle body, use carburetor cleaner to remove the carbon deposits and then recalibrate the throttle opening. Be careful to avoid spraying cleaner into electrical connectors to prevent damage to internal electronic components. 2. For air leaks at the rear of the throttle body, use a smoke leak detector to locate the leak and then repair or replace damaged gaskets or vacuum hoses. 3. For generator failure, use a multimeter to check if the generator's output is within the normal range. If not, replace the faulty generator. 4. For damage to the throttle body or idle speed control valve, replace the throttle body or idle speed control valve. Finally, it is normal for idle speed to be high during a cold start (for a very short duration). This helps the engine reach optimal operating temperature quickly, improving fuel efficiency and protecting the engine. If idle speed remains high after the engine warms up, check for issues like throttle carbon buildup.
