Reasons for a car running rough can include problems with the engine temperature or oxygen sensors, bad hoses, ignition system or timing problems, clogged air or fuel filters, and faulty spark plugs or fuel injectors. Many problems that manifest through a rough idle can be easily fixed.
If the engine temperature sensor fails, it sends the wrong temperature signal, and the result is a fuel mixture that is too lean, which makes the car run rough. As the weather warms up, this is rarely a problem. A failing oxygen sensor can also cause a car to run rough while idling, but it won't affect the car while driving.
A bad hose in any part of the engine is easy to fix. Sometimes, vacuum lines cause a rough idle due to a vacuum leak, and these lines are also simple to replace.
A common cause of an engine running rough is faulty ignition timing. Once the timing is adjusted, the car should run smoothly. The idle speed may also be set too low. Other problems in the ignition system that can cause rough idling include faulty spark plugs, rotors, ignition wires or distributor caps.
Engines have a tough time running smoothly if the fuel or air filters are clogged. These parts are also easy to replace. Dirty fuel injectors must be replaced to keep an engine running well.