I have a very basic question: What is rough idle? Does it mean that when the car is idling, the tachometer is not stable at ~750rpm? My car does not run smooth like a new car, but how can I tell if it has a rough idle?

Thanks

 

People use the term "rough idle" to describe so many different problems that it's becoming useless. Better to describe in detail what it does.

"Surging" idle, "hunting" idle:
Engine runs OK, but the idle is not steady. Engine RPM goes up & down, just as if you were playing with the gas pedal.

Misfiring:
This is a sharper drop in RPM when a cylinder fails to fire. The character of the RPM & the sound of the exhaust is sorta distinctive, not really possible to duplicate by playing with the gas pedal.