Why Does a Car Shake at Idle?
2 Answers
Here are the reasons why a car shakes at idle: 1. Severe engine carbon deposits: The most common cause of car shaking due to engine carbon deposits is a dirty throttle body or excessive carbon buildup in the fuel injectors. When there is too much carbon inside the engine, the gasoline sprayed by the cold-start injector is largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts, making it difficult to start. In this situation, the car only starts easily once the carbon deposits are saturated with gasoline. After starting, the gasoline adsorbed on the carbon deposits is sucked into the cylinder by the engine's vacuum and burned, making the air-fuel mixture too rich. The engine's combustible mixture alternates between being too lean and too rich, causing the idle to shake after a cold start. 2. Ignition system issues: Check the condition of the spark plugs, high-voltage wires, and ignition coils. Problems with the ignition system or poor performance of the spark plugs can also lead to such faults.
My old ride started idling rough after seven or eight years, with the steering wheel vibrating like it was sending Morse code. The main culprit was worn-out engine mounts causing unstable engine positioning – over time, the rubber hardens and loses its cushioning effect. Severe carbon buildup in the throttle body also leads to inconsistent air intake, making the engine run unevenly. Worn spark plug gaps or weak ignition coils can cause misfiring and shaking too. Last time I filled up with off-brand fuel, the vibrations got way worse – poor fuel quality leads to incomplete combustion. You'll need to check these issues one by one; don't skip steps. Fix them properly, and the cabin will quiet down immediately.
