It's the silver box next to the ignition coil with hoses going to it. One
big hose goes to the exhaust, the other goes to the boot just in front of
the MAS. It has a vacuum diaphragm on top of it with a small hose. Take the
two screws out of the bracket holding the diaphragm onto the silver box
(work them back and forth with some lube, they corrode and break easily)
and look inside. If it's all black/burnt looking inside (normally the
plunger is blue rubber..), the reed valves in it are toast and are letting
exhaust go backwards through the valve, up the hose and through the MAS
killing them. If nothing else, "disable" the valve until you can get a
replacement. It's a dealer only part as far as I know.

It's purpose it to allow air into the exhaust for emmisions (instead of
having an air pump...)