I guess you're talking about the PCV system? One end comes off the inlet manifold, other end goes into the oil cooler and the 3rd goes to the hockey puck? It's part of the vacuum system, which draws pressure from the cylinder head and oil cooler, removing fumes etc , basically the breather system for the engine. If this has a leak then it will almost certainly cause rough idle as extra air is drawn into the manifold, also if it's blown before a one way check valve then you'll have a big boost leak. Either fit a catch can setup, a silicone replacement or buy a new set of pipes from audi.
