My audi a4 04 1.8t started idling rough. Based on info I found it sounds like a vac leak. I took the hose apart at the point shown in the picture and capped it off...the idling smoothed out. The other end of the hose just goes to the intake...so I am assuming that "black thing" is some kind of valve. Am I guessing right?

Part number/and or a good place to buy would be cool too.

Thanks guys.

 

http://www.ecstuning.com/Search/06A129101F/
It appears to be the same one. Check the part number on yours to verify to match.

 

Thanks for the link.

I took my valve apart to see how it is made/works. (large pliers, and bandsaw ) Turns out the rubber membrane was cracked.

for future reference:

When working properly this is basically a valve that lets air thru both ways, till it gets under vacuum. Under vacuum the rubber membrane will restrict, but not totally stop air flow. I heard my valve making a hissing noise which is why I suspected it. A good valve will still make a little noise, but if it is enough that you hear it over the engine noise, probably it is bad.