What does unstable idle speed refer to?
3 Answers
Unstable idle speed refers to the fluctuation of the tachometer when the engine is idling, accompanied by engine shaking. There are many reasons for unstable idle speed, including: dirt, wear, or incorrect installation of mechanical parts, leading to changes in the power of individual cylinders, resulting in an imbalance of power among the cylinders and causing unstable idle speed; abnormal engine electronic control system, leading to poor combustion of the air-fuel mixture, making it difficult to balance the power of each cylinder, resulting in unstable idle speed. Idle speed refers to the operation of the engine without any load, only overcoming the internal friction resistance of the components, without outputting power externally. The minimum speed required to maintain stable engine operation is called idle speed. For an engine with good performance, the idle speed is generally between 550 to 800 revolutions per minute.
Unstable idle is what we commonly refer to as the engine shaking violently when running, with the tachometer needle jumping around. For example, when you're stopped at a red light, the car sits there buzzing and trembling, with the steering wheel shaking along, and the RPM fluctuating between 800 and 1200. This issue is mostly caused by a misbehaving component in the engine—it could be the throttle body clogged with sludge, the fuel injectors acting up, or the mass airflow sensor sending incorrect data to the computer. Some older cars with excessive spark plug carbon buildup also behave this way, as the inhaled gasoline doesn't burn completely, and black smoke may even come out of the exhaust pipe. The most annoying part is that fuel consumption will also skyrocket—better take it to a repair shop ASAP.
Unstable idle speed mainly refers to the engine's RPM fluctuating between high and low when idling. After a cold start, or when waiting at a red light with the brake pressed and in D gear, you can clearly feel the car jerking and vibrating, sometimes even shaking so much that the center console rattles. Common issues can be categorized into several directions: excessive carbon buildup on the throttle body affecting its opening and closing angle, leading to inconsistent air intake; poor atomization from the fuel injectors causing incomplete combustion; aging spark plugs or ignition coils resulting in intermittent firing; or a leak in the exhaust pipe causing the engine to miscalculate the air flow. You can simply check if the engine warning light is on and inspect the condition of the spark plugs.
