What causes the car to shake when starting?
3 Answers
The reasons why the car shakes when starting are: 1. Aging engine mounts; 2. Excessive carbon deposits on the throttle body; 3. Poor performance in one of the cylinders. The connection points between the car's engine and the frame are usually equipped with engine mounts, which can absorb vibrations, preventing them from being directly transmitted to the car body. Most engine mounts are made of rubber, and rubber products can age over time. When rubber engine mounts age, they become hard, which can cause the engine to shake during operation. The throttle body is known as the throat of the engine. When the accelerator pedal is pressed, the throttle opening increases, allowing more air to enter the cylinders, and the ECU then adjusts the fuel injection system to inject more fuel. If there are carbon deposits on the throttle body, the engine may experience unstable idling.
I've driven quite a few older cars, and severe shaking during startup is usually due to issues with the ignition system, such as dirty spark plugs or worn electrodes that fail to produce sparks, preventing fuel ignition and causing the entire engine to shake. It could also be a fuel system problem, where carbon buildup in the fuel injectors blocks proper fuel delivery, or an overly dirty air filter restricts airflow, leading to uneven combustion of the mixture. I recall it being more noticeable during cold winter starts—low oil pressure or broken engine mounts can also worsen the shaking. Don’t delay fixing such issues, as they can damage internal engine components, increase fuel consumption, and even cause stalling risks. I recommend checking ignition components and filters monthly and adding fuel system cleaner during seasonal changes to effectively prevent most startup shaking issues.
As someone who has long focused on car maintenance, I've found that startup shaking mostly stems from issues with ignition coils or spark plugs, causing abnormal operation in some cylinders and resulting in uneven engine vibrations. Insufficient fuel supply is also a key factor, such as low fuel pump pressure or dirty fuel injectors leading to unstable fuel delivery. Carbon buildup in the throttle body or faulty air flow sensors can obstruct air intake, disrupting the air-fuel mixture ratio. An aging battery with insufficient power also makes the starter weaker, especially noticeable in cold weather. These causes often indicate neglected routine maintenance, which accumulates and jeopardizes driving safety—increased shaking may lead to engine knocking or excessive emissions. Prompt professional inspection, combined with regular fuel system cleaning and electrical circuit checks, helps maintain the car's healthy operation.
