help me identify this vacuum hose pls
 

The hose right in between the greddy elbow and fire wall. I tried covering the hose with my finger and its definitely pulling vacuum. It still has the hose clamp attached. Where does it hook up to? Was it for the stock elbow intake?

That's the AWS hose, there's a port on the stock elbow for this.

Do people usually plug this up? How will this affect the engine if left unplugged?

Just plug it, the AWS is worthless. Others have used a filter on it. AWS is what shoots you up to 3k rpm on start up

Thanks guys, I'm looking at this fd to buy off this guy in MO. I saw this and wanted to make sure it's not a problem. As you can tell, it's my first FD.

If the car has an aftermarket standalone ecu you can remove the entire system and block it off with block off plates. Much cleaner.

You can also pick up a JDM UIM. It doesnt have the AWS on it. Also, oil can leak out of it too so dont be surprised if you have oil on your firewall.

What happens if you plug it but AWS is still enabled?

Some fd stalls on warm ups and some don't on a factory ecu. If you have an aftermarket ecu, this will not be an issue, like the PFC. You have to deactivate the AWS (depinning or cutting the wire) to be able to run the PFC properly.

If your FD stalls during warm up after plugging the aws pipe, you can drill a small hole on the pipe so it can still suck air in. Of course this makes sense if you have a Greddy Elbow.

Just get some block off plates for 50bux and get rid of it.