Why does the engine shake at idle but not when accelerating?
2 Answers
Engine shaking at idle but not during acceleration is mainly caused by changes in fuel injection quantity, throttle carbon deposits, spark plug failure, and other reasons. The common causes are as follows: 1. Changes in fuel injection quantity: This leads to fluctuating engine speeds, causing the engine to shake at idle. 2. Sensor malfunction: Issues with the fuel pump pressure regulator or sensors can result in faulty fuel injection correction signals. 3. Dirty throttle: Carbon deposits or dirt on the throttle, or incorrect positioning of the throttle position sensor. 4. Spark plug failure: Prolonged use of spark plugs can cause increased spark plug gaps or excessive carbon buildup at the ignition point, leading to poor sparking or insufficient ignition energy, resulting in cylinder misfires, reduced power, and engine shaking.
I've driven for over a decade and encountered many cases of idle vibration with normal acceleration. This usually happens because at idle, the engine runs at low RPM, and issues with the ignition system or intake cause an imbalance in the air-fuel mixture due to a dirty idle control valve, leading to noticeable shaking. Once you accelerate and the RPM increases, the system runs smoothly, and the vibration naturally disappears. Check if the spark plugs or ignition coils are worn out or have carbon buildup—if the contacts are burnt, replacing them can solve the problem. Carbon buildup in the throttle body is another common cause; removing it and cleaning with a spray cleaner will stabilize idle. Don’t forget to inspect the vacuum hoses for leaks, as small cracks can cause unstable idle. Also, pay attention to worn engine mounts in older cars, as deteriorated dampening leads to noticeable vibrations. In short, if you notice anything unusual, get it checked early to avoid poor combustion and engine damage over time.
