If a car shakes when it is idling, the cause is usually something happening with the engine, such as needing new spark plugs. The spark plugs provide an electrical arc that ignites the gasoline vapor in the vehicle to push the piston and provide power to move the car down the road. Even though the vehicle is just idling, it takes power to keep the engine running smoothly.
A wire gauge allows checking the spark gap, when installing new spark plugs. Improperly gapped plugs are sometimes the reason behind the rough running vehicle. The operator's manual for the vehicle provides information on the gap to use.
Prestolite Wire recommends visually checking plug wires while the engine is running to look for any arcing. With the engine off, it is possible to check the condition of insulation, boots and to look for burn marks. All of these are potential causes of the rough idle.
A dirty air filter starves the engine of air. Filters are generally inexpensive and easily changed. Auto manufacturers include recommendations for the interval to change filters. However, operating the vehicle in dusty conditions reduces the time between changes.
With a dirty fuel filter, the vehicle does not receive enough fuel to operate smoothly. Other potential causes of fuel problems include a defective fuel pressure switch or a defective fuel pump.