What Causes Fluctuating Idle Speed in Cars?
2 Answers
The reasons for fluctuating idle speed in cars are as follows: Excessive accumulated dirt: When too much dirt accumulates in the throttle and surrounding intake passages, it changes the cross-sectional area of the air channel. This also prevents the control unit from accurately controlling the intake air volume during idle, resulting in excessively rich or lean air-fuel mixtures and abnormal combustion conditions. Severe engine carbon buildup: When there is excessive carbon buildup inside the car's engine, the gasoline injected by the cold start injector gets absorbed by the carbon deposits, causing the air-fuel mixture to become too lean. During startup, this makes the engine's air-fuel mixture unstable, leading to irregular idle speed that fluctuates up and down.
In my years of driving experience, I've seen it all. Erratic idle speed often occurs when the throttle body is dirty. Thick carbon buildup blocks airflow, preventing the engine from drawing in air properly, causing the RPM to fluctuate. Alternatively, it could be an issue with the ignition system—like worn-out spark plugs failing to ignite, leading to unstable cylinder pressure and erratic behavior. A dirty or inaccurate mass airflow sensor can also disrupt the air-fuel ratio, causing the ECU to malfunction. I recommend first checking if the spark plug gap has widened and cleaning the throttle body. Don’t overlook a clogged air filter either, as insufficient airflow can cause similar issues. Unstable idle not only affects fuel efficiency but also causes vibrations. Address it early to prevent engine damage. While DIY part replacements are manageable, complex issues are best left to professionals. Regular throttle body maintenance helps prevent carbon buildup.
