A faulty spark plug may cause your car to shake when idling. A bad spark plug can cause uneven fuel consumption during the combustion process. In such a situation, your vehicle’s RPM will fluctuate, and the engine will make a lot of noise. In addition to fluctuating RPM and loud noise, the force the engine is exerting to complete the combustion cycle can cause the car's engine to start vibrating due to damaged spark plugs. The vibrations can spread from the engine to any part of the vehicle.