The rubber tube on the back of the elbow, slightly larger diameter than the idle control valve hose, is a breather hose from the oil tank. There are actually two hoses back there that are the same size. One hose is the aforementioned hose that connects to the rubber elbow and the other hose connects to a breather neck on top of the engine case.
The reason the oil tank needs to breathe is because the breather neck on top of the engine case spits positive air pressure (and oil) into the oil tank. That positive air pressure in the oil tank needs to be relieved. So that relief is plumbed right back into the throttle body of the engine, via the connection to the rubber elbow.
For performance/racing purposes, this breather to the elbow is often disconnected because you don't want to feed your engine oil vapor (or mostly oil, especially if overfilled) and burn it. So the breather hose is often connected to a "catch can" like this
http://forums.pelicanparts.com/porsche-911-technical-forum/528415-can-i-see-oil-vent-catch-cans.html |
