What are the causes of unstable hot idle speed fluctuating between high and low?

[McRylee]

The causes of unstable hot idle speed fluctuating between high and low are: 1. Oil accumulation and carbon deposits in the throttle body, dirty intake passages; 2. Weak spark due to faulty spark plugs; 3. Fuel system blockages represented by clogged fuel injectors, intake pipe malfunctions; 4. Related sensor failures; 5. Poor valve sealing, intake manifold leaks; 6. Incorrect ignition timing (too early or too late); 7. Improper idle speed adjustment. Solutions for unstable hot idle speed fluctuating between high and low: 1. Clean carbon deposits from the engine and idle speed control valve; 2. Clean the fuel system; 3. Replace spark plugs, high-voltage wires, and ignition coils; 4. Clean the air filter and throttle body; 5. Replace aged engine components.

[AndrewAnn]

I've encountered unstable hot idle speed fluctuating up and down many times. The most common cause is excessive carbon buildup in the throttle body. When the engine heats up, the carbon deposits block the intake, causing the ECU to lose precise control over idle speed, making the RPM jump around. Sensor issues are another culprit - a dirty or faulty mass airflow sensor can't accurately report intake air volume to the computer, leading to unstable RPM. Worn spark plugs also cause trouble by disrupting ignition timing, directly affecting idle control. Clogged fuel injectors creating insufficient fuel supply can trigger this problem too. Don't overlook vacuum leaks either - extra air intake disrupts the balance. These minor issues not only affect driving smoothness but may damage engine longevity if left unfixed. Get to a repair shop promptly for cleaning and inspection before it leads to roadside stalling.

[EvangelineMarie]

Erratic idle speed occurs during the warm-up phase, which is the most frustrating thing when I'm driving. It's mostly caused by sludge buildup in the throttle body. When the engine warms up, carbon deposits become more active, blocking airflow and causing ECU adjustment errors that make the RPM fluctuate. Dirty air flow or oxygen sensors sending inaccurate data can also lead to uncontrolled idle speed. Vacuum system leaks are even more common – aging hoses with air leaks create intake imbalance, making the RPM swing. Worn spark plugs causing intermittent ignition can also make the idle speed jump. These faults make the car shake severely during startup or stopping, affecting safety. I suggest first checking the hoses for obvious cracks or leaks. Changing the air filter more frequently during maintenance can prevent carbon buildup. However, since electronic components are complex, it's better to leave a thorough inspection to professional technicians.

[Mallory]

Unstable hot idle, fluctuating up and down, has many root causes. Carbon buildup in the throttle body blocks air intake, causing ECU control malfunction; dirty air flow sensor provides incorrect data; vacuum tube leaks lead to excessive air intake; old spark plugs cause unstable ignition; clogged fuel injectors result in insufficient fuel supply. Carbon deposits are more likely to cause problems when the engine is hot. Regular maintenance can prevent this, such as cleaning the system every 5,000 kilometers. Ignoring idle issues can damage the engine, so repair it early.