What Causes Engine Idle Speed to Fluctuate Unsteadily?

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Strictly speaking, unstable idle speed generally refers to fluctuations exceeding ±20r/min of the expected idle speed or severe speed oscillations. Below are the reasons why engine idle speed fluctuates unsteadily: 1. Oil accumulation and carbon deposits in the throttle body, or dirty intake passages: Blockages in the intake passages can cause idle speed fluctuations, while carbon deposits or oil in the throttle body are the most common causes of unstable idle speed. Carbon deposits in the throttle body affect its opening, disrupting the air intake volume, leading to erratic engine speed fluctuations during cold starts, accompanied by strong vibrations. Severe oil accumulation or carbon deposits, including dirty intake passages, can make it difficult to start the car in cold weather. Cleaning the throttle body is necessary and is considered a routine maintenance task. 2. Weak spark due to faulty spark plugs: Spark plugs can also accumulate carbon deposits, which can be cleaned to restore their activity. However, some spark plugs nearing the end of their lifespan may experience performance degradation. Weak sparks from individual spark plugs in certain cylinders can affect car performance. Faulty spark plugs may cause "misfire" symptoms, but minor faults in individual cylinder spark plugs can lead to unstable idle speed. Therefore, cleaning or replacing spark plugs is also a viable solution when the car exhibits unstable idle speed. 3. Fuel system blockages (e.g., fuel injectors) or intake pipe faults: Blockages in the fuel system, such as clogged fuel injectors, can result in insufficient fuel pressure, poor fuel atomization, and imbalanced air-fuel mixture, leading to incomplete combustion and related issues. Leaks in the intake pipe can affect the accuracy of the air flow sensor readings, causing the ECU to inaccurately control the air intake volume, further disrupting the air-fuel mixture and resulting in unstable engine idle speed. Cleaning the fuel system is another maintenance task, often referred to as an "engine overhaul." Fuel system issues can also lead to excessive emissions, causing the car to fail annual inspections. 4. Faulty sensors: Malfunctioning sensors can prevent the ECU from accurately controlling the air-fuel ratio. Common examples include faulty coolant temperature sensors, oxygen sensors, and throttle position sensors. The coolant temperature sensor is typically installed on the engine block or cylinder head water jacket, directly contacting the coolant to measure temperature and provide reference data to the ECU. A faulty coolant temperature sensor can lead to increased fuel consumption, unstable idle speed, and abnormal engine speed. Similarly, faults in the throttle position sensor or front oxygen sensor can cause incorrect air-fuel ratios, leading to incomplete combustion and related issues. In such cases, replacing the faulty sensors is necessary.