So my R53 has just recently started idling pretty rough on start, and even sputtering a little in the first few seconds after it starts. No problems actually getting it to start up, but it immediately sputters a little (RPMs drop to probably 500, then right back up). It then idles a little rough for a bit and ultimately smooths out. This is a new development. I recently took some steel wool to the #3 ignition coil (and checked the others). The spark plugs are new as of last October, but the coil is stock. Car has 72k on it, and a 15% WMM pulley. I plan to pull the intake house and see if the throttle body is dirty, in part because I feel like the throttle has been a little "sticky" lately (slower to respond). I wonder if the body has gotten dirty and is causing the flap/valve to stick. Any other ideas?

 

Common issue. Some suggest replacing the PCV valve can be a fix. Some suggest getting a new ECU flash from the dealer can do the trick. I've experienced this sporadically myself and cannot pinpoint it. Common wisdom would have you question the ignition system - and sometimes you can look at your coil and find a badly oxidated post (usually 3 or 4). Good luck. There are several threads dedicated to this issue.