What Causes Unstable Engine Idling in Cars?
2 Answers
Unstable engine idling in cars can be caused by carbon buildup in the throttle body, faulty spark plugs, fuel line blockages, among other issues. Below are detailed explanations: Carbon Buildup in the Throttle Body: Carbon deposits or oil accumulation inside the throttle body are among the most common causes of unstable idling. Carbon buildup affects the air intake volume, leading to fluctuating idle speeds and noticeable engine vibrations, especially during cold starts. Severe carbon deposits or oil in the throttle body, including dirty intake passages, can make the car difficult to start in cold weather. Cleaning the throttle body is necessary and is considered a routine maintenance task. Faulty Spark Plugs: Spark plug issues or carbon deposits can often be resolved by cleaning to restore their functionality. Weak sparks from spark plugs in certain cylinders can impact the car's performance. Faulty spark plugs may cause misfires, and minor issues with individual spark plugs can result in unstable idling. Fuel Line Blockages: Blockages in the fuel system, particularly in the fuel injectors, can lead to insufficient fuel pressure and poor fuel atomization. This disrupts the air-fuel mixture ratio, causing incomplete combustion and resulting in unstable engine idling.
I have an old car, and the idle speed often fluctuates up and down, which feels really uncomfortable. Last time I encountered this issue, it was due to a dirty throttle body with too much carbon buildup, and it improved a lot after cleaning. It could also be a faulty idle control valve or actuator, or something like aging spark plugs affecting ignition. Sometimes sensors like the oxygen sensor or mass airflow sensor failing can confuse the engine computer. Vacuum hose leaks are another common culprit, especially in older cars where rubber tends to deteriorate. I suggest checking the simple things first, like whether the air filter is clogged, before taking it to a shop for diagnosis. Unstable idle not only affects driving but also wastes fuel and can cause stalling when stopping—it's best not to ignore it.
