It seems like there's something missing from the engine compartment of my
wife's '97 Corolla (180K), but I need help identifying it. The part must
have just fallen off! We've had the car since it was less than a year old,
and no one has ever worked on it other than a timing belt change at the
dealer five years ago (yes, I know it's due again) and the regular oil
changes I do at home. I never had to work on the engine, so I never had to
familiarize myself with it until now.

The part I'm asking about is the round object just forward of where the A/C
lines come through the firewall, and just behind the dipstick. There's a
shiney screw hole in the intake manifold nearby, which I suppose should have
clamped the missing part in place. See the photo:

http://s43.photobucket.com/albums/e382/bctucker/?action=view&current=WhatIsIt_97
_Corolla.jpg

Could someone please advise me what that round can is? And what's missing
from the top of it? If it's a junk yard item, what should I ask for?

Thanks for any advice you can offer!