Why does an automatic transmission car shake when idling in D gear?
2 Answers
The reasons for an automatic transmission car shaking when idling in D gear are as follows: 1. The engine speed is too high, such as when the engine speed is too high during cold start. At this time, the torque converter transmits more power, resulting in greater impact force when shifting gears and more noticeable shaking. 2. The brake is not pressed firmly enough. When shifting into gear, the engine power is directly transmitted to the wheels. If the brake is not pressed firmly, the wheels may slightly move when shifting into D gear, causing the car to suddenly move forward and creating a stronger shaking sensation. 3. Releasing the brake too early. This is most common when waiting at a red light. After the light turns green, shifting into gear and releasing the brake to start, the car suddenly jerks forward when the brake is released. This is caused by releasing the brake too early, leading to abrupt power engagement. Additional Information: 1. It is normal for an automatic transmission car to have slight shaking when shifting into D gear, especially for AT transmissions equipped with a torque converter. This phenomenon is more noticeable. In cars with a torque converter, the engine power is transmitted to the transmission input shaft through the torque converter when in neutral. When shifting into gear, the transmission engages while the input shaft is rotating, causing an impact. 2. Some automatic transmissions use multi-plate clutches or dry clutches to transmit power between the engine and the transmission. When shifting gears, the clutch is completely disengaged, so there is no impact force, making it less likely to notice any shaking.
I've always driven automatic transmission cars, and when encountering idle shaking in D gear, I found the most common cause is excessive engine carbon buildup. Carbon deposits can lead to uneven cylinder compression, especially under the increased load in D gear, making the engine struggle for breath and prone to shaking. Another major reason is aging spark plugs causing unstable ignition—my old car became much quieter after replacing the spark plugs. Worn engine mounts are also problematic; they're supposed to absorb vibrations, but when cracked, the shakes transmit directly into the cabin. Checking these points is simple: start by cleaning the throttle body or using fuel additives during highway driving. If the shaking is severe, it might be a torque converter engagement issue—then you should visit a professional shop to check the data stream with a diagnostic tool, avoiding delays that could damage more components. In short, regular maintenance can prevent 90% of issues, and always remember to change the engine oil on time.
