What Causes Unstable Car Idling?
2 Answers
There are several reasons for unstable car idling, including severe engine carbon buildup and excessive carbon deposits in the throttle body and intake manifold. The details are as follows: Severe Engine Carbon Buildup: When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold start injector will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts and making it difficult to start the engine. In this situation, the engine will only start easily once the carbon deposits have absorbed enough gasoline to become saturated. After starting, the gasoline adsorbed on the carbon deposits will be drawn into the combustion chamber by the engine's vacuum suction, causing the air-fuel mixture to become overly rich. The combustible mixture in the engine alternates between lean and rich, leading to unstable idling after a cold start. It is necessary to clean the fuel system and inspect the idle air control valve. It is recommended to use high-quality fuel to reduce carbon buildup. Excessive Carbon Buildup in the Throttle Body and Intake Manifold: Excessive carbon deposits and dirt in the throttle body and surrounding intake manifold change the cross-sectional area of the air passage, making it difficult for the control unit to precisely regulate the idle air intake. This results in an overly rich or lean air-fuel mixture, leading to abnormal combustion. Spark plugs can also accumulate carbon deposits, which can be cleaned to restore their activity. However, some spark plugs may be nearing the end of their service life, leading to reduced performance. Weak sparks from spark plugs in individual cylinders can affect the car's performance. Faulty spark plugs can cause misfires, but minor faults in spark plugs of individual cylinders may result in unstable idling. It is necessary to regularly maintain the car at an auto shop for thorough and professional inspection and repair. Regularly clean the throttle body and intake system to remove carbon deposits.
After driving for so many years, I've encountered unstable idling several times, and each time it was nerve-wracking. When parked, the engine would shake and sputter, as if gasping for breath. The most common causes are aging spark plugs, where inaccurate ignition leads to significant RPM fluctuations; a dirty air filter clogging the intake, preventing the engine from getting enough air; a carbon-clogged idle control valve sticking and failing to regulate RPM properly; vacuum line leaks causing incorrect air-fuel mixture; or fuel system issues, such as clogged fuel injectors leading to unstable fuel supply. Sensor problems, like a faulty throttle position sensor sending incorrect signals, can also mislead the car's computer. I remember one time when a spark plug failed, and replacing it myself made a big difference. Don’t take safety lightly—unstable idling can escalate to stalling, especially at intersections. Regular maintenance checks on these components are recommended to catch issues early, saving both hassle and money.
