Why does the car shake when idling?
2 Answers
Reasons for engine idle shaking: 1. Engine intake pipe leakage: The intake system contains many intake manifolds or various valves. Leakage in these components allows unwanted gases to enter, causing the concentration of the air-fuel mixture in the intake manifold to become too high or too low, ultimately leading to intermittent engine shaking or weak idle performance. 2. Fuel system blockage or malfunction: Due to a blocked fuel tank vent, clogged fuel tank valve, fuel filter, or fuel lines between the tank and the carburetor, the engine may not receive the required amount of fuel during idle, resulting in a lean mixture, reduced engine power output, and idle shaking. 3. Idle control valve failure: The idle operation of an electronic fuel injection engine is mostly controlled by the idle control valve. The ECU adjusts the opening of the idle control valve based on signals such as engine speed, temperature, and throttle position to maintain stable idle speed. If the valve malfunctions, it can cause uneven air distribution, fluctuating idle power, and vehicle shaking. 4. Engine carbon buildup: This includes carbon deposits in the cylinders, throttle body, spark plugs, and intake system—areas critical to engine power output. Excessive carbon buildup reduces ignition energy, decreases intake efficiency, and ultimately leads to unstable engine power output, idle shaking, and poor acceleration.
I've been repairing cars for over a decade and have encountered many cases of engine idle shaking. The main issues usually lie in the ignition system or engine mounts. At idle, the engine runs at very low RPMs. If the spark plugs are carbon-fouled or worn out, the ignition becomes uneven, leading to increased vibrations – especially noticeable in older vehicle models. Another common problem is worn engine mounts. These components absorb vibrations, but if their rubber parts age and crack, the entire engine shakes noticeably, particularly at idle. Fuel system issues shouldn't be ignored either – clogged fuel injectors causing unstable gasoline supply can result in severe idle shaking. Some minor factors like vacuum leaks or dirty throttle bodies can also cause unstable idling. The key is to conduct a thorough inspection from the beginning. The safest approach is regular maintenance including spark plug replacement, and visiting a professional shop for diagnosis at the first sign of shaking – don't wait for minor issues to turn into major repairs.
