I just picked up a 2009 128i with 80,000 KMs. The previous owner lived on a farm, and most likely ran farm gas in there which isn't filtered as good as the gas station gas. I have noticed that at idle there the engine is not exactly smooth. There is just a bit of roughness to it.
I am going to replace the spark plugs, and I want to replace the fuel filter, but I have heard that there is a "permanent fuel filter" and that it is in the tank. I think there has to be a way to replace it, and I am hoping that someone here might have done it before, or have some info.
I have ran some sea foam in the tank to clean things out a bit that helped a little. I have seen fuel filters cause a rough idle before, and new plugs can't hurt. I can't believe that car companies put filters in the tank