Why does the car shake at idle?
3 Answers
The reasons for car shaking at idle are: 1. Excessive carbon deposits. Severe carbon buildup inside the engine cylinders can cause spark plug blockage and valve blockage; 2. Fuel supply system failure. Unstable oil pressure can easily lead to unstable mixture concentration; 3. Aging engine mounts; 4. Issues with the ignition coil or spark plugs failing to fire for some reason. Idle refers to the engine running without any load, only overcoming the frictional resistance of its internal components, without delivering external power. The minimum speed required to maintain stable engine operation is called idle speed, which is one of the five basic operating conditions of an engine. A well-functioning engine typically has an idle speed of 550 to 800 revolutions per minute. The idle speed can be adjusted by altering the throttle opening size, idle fuel supply, etc.
I've been driving for over a decade and often encounter idle vibration issues. The most common causes are aging spark plugs or faulty ignition coils, where weak sparks during ignition lead to unstable engine RPM and vehicle shaking. Another reason could be incorrect air-fuel ratio, such as a dirty mass airflow sensor, clogged fuel injectors, or intake manifold leaks, which can also cause vibrations. Carbon buildup is another major problem—frequent low-speed city driving leads to accumulation, and a dirty throttle body is even harder to clean. If the engine mounts are worn out, their shock absorption weakens, making vibrations more noticeable. I recommend first checking yourself: inspect the ignition coils and spark plugs for wear, or visit a professional shop for a cylinder compression test and throttle body cleaning. Don’t ignore vibrations—timely repairs save fuel and improve safety, as shaking at high speeds can be even more dangerous.
When I repair my car myself, I find that idle shaking is mostly caused by ignition system issues. Spark plugs can wear out or accumulate carbon deposits over time, leading to insufficient ignition energy and uneven cylinder pressure, which makes the car shake. Fuel system problems, such as clogged fuel injectors or unstable fuel pressure, can also cause similar symptoms. A failing fuel pump or dirty fuel filter can result in poor fuel injection. Carbon buildup in the intake manifold and combustion chamber affects the uniformity of the air-fuel mixture. Cracked or loose engine mount rubber blocks lose their damping effect, making the shaking more noticeable. Possible related causes include excessive load from running the air conditioning or indirect effects from transmission issues in automatic cars. For DIY troubleshooting, try disconnecting the battery negative terminal to reset the computer system. Using a diagnostic tool to read data streams provides more accurate results. If all else fails, take the car to a repair shop for a comprehensive inspection.
