What Causes Unstable Idle?

[DelChloe]

Reasons for unstable idle: 1. Idle switch not closing; 2. Faulty idle control valve; 3. Air intake manifold leakage; 4. Incorrect valve timing; 5. Fuel injector dripping or clogging. Troubleshooting methods: 1. Adjust, repair or replace the throttle position sensor; 2. Clean or replace the idle control valve, and use a special decoder to perform basic setting of idle speed; 3. Locate the leakage point, reseal or replace corresponding components; 4. Check timing marks and readjust valve timing according to standards; 5. Clean fuel injectors, inspect each injector's fuel delivery and confirm for any clogging or dripping.

[Steven]

When I was younger, I also encountered idle instability issues. The car would suddenly shake or the RPM would fluctuate when waiting at a red light, which was quite unsettling. Common causes include excessive carbon buildup on the spark plugs leading to poor ignition, or a clogged air filter preventing the engine from getting enough air. Also, if the idle control valve is aging, that thing can get stuck, and the RPM won't be controlled accurately. Severe engine carbon buildup can also cause idle instability. My car was like this before, and it improved a lot after replacing the spark plugs and cleaning the throttle body. Additionally, if sensors like the oxygen sensor fail, the ECU computer can't make accurate judgments, and the idle will jump around. Regular maintenance checks on these areas can save a lot on repair costs and avoid unexpected stalling while driving.

[SanBlakely]

Unstable idle speed is mostly related to the intake and fuel system. If the spark plugs are dirty or burnt out and fail to ignite, the engine speed becomes unstable. Also check the idle air control valve - sometimes it gets clogged with sludge or wears out, causing the RPM to fluctuate erratically like dancing. If the fuel injectors are clogged with carbon deposits restricting fuel supply, the idle will also fluctuate. Sensors like the throttle position sensor can cause issues too - if it malfunctions, the ECU gets confused and idle becomes hard to control. I once experienced this in my car and solved it by replacing the spark plugs. Regular throttle body cleaning and air filter replacement can prevent such issues - don't wait until major problems occur to fix them.

[DiJack]

Last time my car was idling very rough, the mechanic said it was a faulty ignition coil. Unstable fuel supply from the fuel pump can also cause RPM fluctuations, and a dirty air filter blocking the intake can lead to idle issues too. Faulty idle valves or sensors are also common culprits. I suggest first checking the air filter and spark plugs yourself - it's cost-effective and easy. If the problem persists, visit a professional shop promptly to avoid stalling risks.