If your engine shakes while idling this is a telltale sign that the engine is misfiring on one or more cylinders. This may be as simple as spark plugs or spark plug wires but may also be a more serious internal engine problem like a burned valve. Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine’s power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle.