What are the reasons for high idle speed when starting a car?

[Harold]

There are many reasons that can cause high idle speed in a car. If high idle speed occurs, the focus should be on the engine ignition and intake system. Below are the reasons and solutions: 1. Idle speed control valve malfunction: The normal idle speed of an electronically fuel-injected engine is determined by the idle speed control valve. The electronic control unit (ECU) calculates signals such as engine speed, temperature, throttle switch, and air conditioning switch, then adjusts the idle speed control valve to open the intake bypass channel or directly increase the throttle opening to enhance air intake and raise the engine idle speed. If oil contamination or carbon deposits cause the idle speed control valve to stick or the throttle valve to not close properly, the ECU cannot accurately adjust the engine idle speed. Solution: Replace or repair the idle speed control valve. 2. Intake system leakage: Normally, the air intake must be strictly controlled. If there is a leak in the intake pipe, the air flow sensor cannot measure the actual intake air volume, leading to inaccurate air intake control by the ECU and unstable idle speed. Solution: Inspect and repair the intake system. 3. Fuel injector leakage or clogging: If fuel injectors leak or become clogged, they cannot spray fuel according to the ECU's instructions, resulting in an overly rich or lean air-fuel mixture, causing poor performance in individual cylinders and unstable idle speed. Solution: Clean or repair the fuel injectors.

[ArcherLee]

I remember last time when the car started, the idle speed remained high for a long time. After struggling for a while, I found out it might be due to a dirty throttle valve getting stuck, with excessive carbon buildup causing unstable air intake, which made the ECU misjudge and increase the RPM. Another common issue is air leaks in the intake system, like aging pipes with cracks sucking in extra air, causing the RPM to spike. Sensors can also be troublemakers—if the coolant temperature sensor fails and thinks the engine is still cold, it will aggressively heat up and raise the RPM. Or if the idle control valve malfunctions, it simply can't adjust the speed properly. Sometimes, having the AC on can also pull up the RPM, and if it doesn’t drop after the engine warms up, it’s troublesome. Regular maintenance shouldn’t be neglected—changing the air filter more often can prevent carbon buildup issues. Otherwise, high idle speed isn’t just noisy but also wastes fuel, making driving particularly unpleasant. Pay attention to idle speed changes after startup, and address any abnormalities early to save trouble.

[LillianAnn]

I once experienced a situation where the idle speed was unusually high right after startup, with the tachometer soaring above 1500 RPM, accompanied by a loud humming noise while driving. After inspection, it turned out to be caused by an intake air leak—a small crack in the vacuum tube made the ECU mistakenly draw in more air, increasing the RPM. The mechanic mentioned that a poor connection in the throttle position sensor wiring could also cause this issue, as erratic signals would lead to inaccurate control. Additionally, a faulty coolant temperature sensor falsely reporting low temperatures could force the engine to maintain high idle speeds. While higher idle speeds during cold starts are normal to protect the engine, persistently high idle when the engine is warm indicates a problem. It's advisable to regularly check the engine bay for loose connections and replace aging rubber hoses as a preventive measure. Driving with high idle speeds not only risks engine damage but also wastes fuel.

[Tabitha]

Start with the basics when checking high idle speed. After starting, listen if the engine sound is smooth; instability may indicate valve or sensor issues. Check for air leaks around the air filter cover or loose pipes yourself, and try unplugging/replugging sensor connectors to reset. Common causes include air leaks or throttle body carbon buildup affecting airflow, prompting ECU compensation by raising RPM. Faulty temperature control can also falsely elevate idle speed. For DIY, first turn off AC to see if RPM drops. If not, professional diagnostic tools are needed—don't delay to prevent minor issues from becoming major.