Why Does an Automatic Transmission Car Shake When Starting?

[ODelaney]

Automatic transmission cars may shake during startup due to the following reasons: 1. The throttle is too dirty, causing uneven air intake and resulting in shaking. 2. Poor fuel delivery, leading to insufficient fuel supply at low throttle, causing uneven operation among engine cylinders and shaking. 3. Abnormal transmission operation. The car shakes during startup but runs normally afterward. 4. Broken or burnt high-voltage wires or spark plug failure, causing some cylinders to misfire and resulting in uneven engine operation and shaking. 5. Check if the battery voltage is within the specified range. The most common causes of car shaking are a dirty throttle or excessive carbon buildup in the fuel injectors. When there is too much carbon buildup inside the engine, the gasoline sprayed by the cold-start injector is largely absorbed by the carbon deposits, leading to a lean air-fuel mixture during cold starts and making it difficult to start. In this situation, the car will only start easily once the carbon deposits are saturated with gasoline. After starting, the gasoline adsorbed on the carbon deposits is drawn into the cylinders by the engine's vacuum suction, enriching the air-fuel mixture. This causes the engine's combustible mixture to fluctuate between lean and rich, resulting in idle shaking after a cold start.

[DeBrielle]

I've encountered this issue several times. I remember last winter when starting the car, the shaking was particularly noticeable—I could feel my whole body vibrating in the seat. It's most likely a problem with the ignition system, such as old spark plugs with heavy carbon buildup or faulty coils, causing the engine to misfire and shake. Transmission-related issues like low fluid levels or internal component wear can also lead to this, especially in cold weather when fluid viscosity changes, making the shaking more frequent. It's advisable to wait a few seconds before starting to let the system warm up. If the problem persists, take it to a repair shop to check and replace any faulty parts. Don't underestimate this issue—prolonged neglect could lead to more severe mechanical damage and affect driving safety.

[SanAdeline]

Vibrations can originate from multiple sources. I've handled similar cases where they often result from aged engine mounts losing their damping effect, or insufficient transmission oil leading to inadequate lubrication, causing vibrations to directly transmit into the cabin. Other factors like a clogged air filter or unstable fuel pump pressure can also cause unstable startups. During maintenance, it's crucial to regularly inspect these components and replace consumables such as engine oil and spark plugs. Prevention is the best strategy, as delaying repairs may exacerbate internal wear and even lead to breakdowns on the road, posing safety hazards.

[DeRonan]

Don't panic when encountering startup vibrations. From personal experience, it might be due to insufficient battery power affecting the supply system or issues with the starter motor. Try gently pressing the accelerator to check the response. If it still vibrates at idle, inspect the maintenance status of related components. Develop good habits like warming up briefly before starting, avoiding sudden acceleration to reduce vibrations, and promptly addressing any abnormalities during daily use to avoid major repair costs.