Why does an automatic transmission car shake when releasing the brake to start?

[DeHenry]

It is due to excessive carbon buildup in the throttle body. Below is relevant information about the throttle body: 1. Function: The throttle body is a controllable valve that regulates air intake into the engine. After entering the intake manifold, the air mixes with gasoline to form a combustible mixture, which then burns to generate power. Connected to the air filter at the top and the engine block at the bottom, it is often referred to as the throat of a car engine. 2. Types: There are two main types of throttle bodies - traditional cable-operated and electronic throttle bodies. In traditional engines, the throttle control mechanism operates via a cable (flexible steel wire) or linkage rod, with one end connected to the accelerator pedal and the other to the throttle linkage plate.

[AlexanderRose]

My car had the same issue before, and it turned out to be caused by severe carbon buildup in the engine. Especially for cars frequently driven in urban areas, low-speed driving is particularly prone to carbon deposits. The shaking was especially noticeable right after releasing the brake, but it improved a lot after getting the intake system cleaned at the repair shop. Another possibility is worn-out spark plugs. My previous car, which hadn't had its spark plugs changed for 100,000 kilometers, experienced starting shakes due to poor ignition. Insufficient or deteriorated transmission fluid can also cause this—I encountered it once when I delayed maintenance. Also, if the engine mounts are worn out, it feels like the whole car is vibrating, but it became quiet immediately after replacement. Anyway, don't ignore shaking issues; they might cause more damage if left unchecked.

[McArcher]

I've researched this issue, and it's mainly related to powertrain matching. When the brake is released, the engine load suddenly increases, and if the engine itself isn't running smoothly, this can happen. For example, if the throttle body is dirty and air intake is insufficient, the ECU needs to readjust the air-fuel ratio; or if the fuel injectors are clogged, causing poor atomization. Worn spark plug gaps can also affect combustion efficiency—I replace mine every 40,000 kilometers. Additionally, damaged engine mounts are most easily overlooked, as aged rubber loses its cushioning effect. Oil that's too thick or of the wrong grade can also increase cold-start resistance. Regular maintenance is crucial—I strictly change my oil every 5,000 kilometers, and vibration issues rarely occur.

[AmeliaFitz]

This issue often occurs during cold starts. I encounter it every morning when driving, and later found out it was a problem with the vacuum booster pump. Prolonged braking consumes vacuum pressure, and the sudden change in intake manifold pressure when releasing the brake causes shaking. The viscosity of transmission fluid is also crucial, especially in winter when low temperatures reduce fluidity, leading to slower oil pressure buildup and prolonged clutch plate semi-engagement. I've noticed that warming up the car for two minutes beforehand helps significantly. If the steering wheel also shakes, it's important to check for brake disc warping. Now I've developed a good habit: shifting to neutral (N) when stopped at a red light for more than 20 seconds, which both protects the transmission and prevents shaking.