so what exactly u mean by rough idle? is it going up and down, on idling? check both things also check the intake pipes sometimes even a small pipe is loose then it cause the trouble, also i am having a sort of same prob in my car, but i figured out its have something to do with fuel pump, when its less than quater gas then my idling goes rough, and once my intake pipe was loose in my bmw it was causing the same stuff so do u check both, just put some premium gas and see if it helps ...