What's this little hose?

[Seesure]

Gentlefolk, I'm hoping someone can advise what this little hose is that's circled in yellow...

I'd just disconnected the battery as I'm away for 5 weeks and I know it will be flat in 3 if I left it connected.

As I was just about to close the bonnet I noticed the little hose was disconnected and so I slid it on to the yellow plastic connector which seemed the most obvious.

As I was on the way to the airport I took a couple of snaps but am curious to know what the hose is for.

The car is a 2003 C200 Kompressor W203 and to be honest I only use it to run around when my main car is worked on by my indy...which seems to be more and more these days....

 

[Wighty]

Egr I believe buddy

 

[LostKiwi]

My thoughts too. Just reconnect it.

 

[DREAMER NO2]

So it is - they go together like mom and dad .

 

[umblecumbuz]

.. or just plug it to see what happens.
If it's vacuum operated egr you might get a performance boost!

 

[Oldspanners]

I think it's the vacuum operation for the air injection system which injects air into the exhaust on start up to help the O2 sensors. As it only works for a short period on cold start it won't make a big difference but obviously needs reconnected as you show.

 

[Seesure]

Many thanks all, I did connect it back together, but was curious as to what it was especially as the car had been put through an MOT a week or so before hand. Not back in the UK until late September but will keep an eye on it to make sure it doesn't part ways again...

 

[Bay Leaf]

I wonder why it was disconnected in the first place