Cars idling rough at stops can have many causes, including damaged sparkplugs, improperly installed sparkplugs or damage to the car's fuel injector, carburetor, vacuum hoses or ignition wires. It is important to take a car to a certified mechanic soon after it begins idling roughly in order to prevent more expensive damage.
Damage to the car's carburetor, vacuum hoses and ignition wires can all lead to the car's engine misfiring. This misfiring can lead to the car idling roughly. Damaged or dirty fuel injectors can cause the same problem. Over the lifetime of the car, the fuel injector can collect a variety of dirt and grime. This dirt and grime can be removed through the use of a fuel system cleaner, but if the user neglects using this cleaner, the fuel injector's problems will grow. Eventually, it will be too dirty or damaged to be cleaned through a fuel system cleaner, meaning that it will need to be replaced to ensure that it doesn't keep the engine from idling roughly.
Rough idling can be relatively cheap to fix, so users should request a diagnostic from a mechanic as soon as they suspect the problem. Mechanics may also provide a fuel system cleaner service as part of oil changes, which is another way to either fix a rough idle or prevent a future one.