Rough idle while car engine is running is an indication of engine misfiring on one or more cylinders, when the engine misfire this can cause unbalance to car engine which then causes rough idle.
Car Problem Diagnosis
The real problem here is that, why the car engine misfire? Engine misfiring can cause by a lot of factors and it is very difficult to test. However, there are only two common reasons that usually causing the problem, first is a vacuum leak and the second is a bad spark plugs or spark plug wires.
There is other reason but commonly encountered on high mileage cars, which is a burned or out of adjustments valve. When this problem is present the affected engine cylinder will encounter low compression thus misfiring occurs. However, if the car is fuel injection the possibilities of clogged fuel injector can also cause engine misfiring. This will happen if the fuel injector is clogged the engine cylinder affected by clogging will be starved of fuel and rough idle can be notice.
Possible Cause of Engine Rough Idle
Vacuum leak, bad spark plug or wires, burn out valves, out of adjustment valves.
How to Fix Engine Rough Idle
First try to replace the spark plug and spark plug wires, usually this fix the problem of rough idle. Refer to your car service manual that covers the car model to know how to replace the spark plug or spark plug wires or bring your car to a reputable auto repair shop and let the mechanic to do the job including inspection to confirm the cause of rough idle car engine.