it is from the oxygen pump. air pump. it is to add clean air to the exhaust gas to fool the guys at the emmisions places into thinking your car has good emissions.
the pipe is called the manifold vacuum control tube its connected to the air pump and pumps air into the exhaust system..its part of the air injection system read info. below.
The air injection system is designed to introduce outside air into the exhaust stream to assist in burning the gases produced by the engine. Air is injected into the exhaust system at one of several locations including the cylinder head, exhaust manifold, or directly into the catalytic converter. Outside air is drawn by an air pump (sometimes referred to as a smog pump), a pulse air valve, or a reed valve. The air pump is run by a fan/accessory belt on the front of the engine or, in some cases, a small electric motor. The pulse air and reed valves work by sensing the drop in manifold vacuum produced by an exhaust valve closing. Each time the drop in vacuum is noticed, a "puff" of fresh air is pulled into the exhaust stream.
Some related parts of an air injection system can include, but are not limited to: air pump, check valve(s), gulp valve, diverter valve, air supply tube, and/or an air manifold.
If you have stock cats, it's nice to have the extra air injection from the A.I.R. pump. Some aftermarket cats say they dont need extra oxygen [from an AIR pump] to function properly.
If you have no cats, it wont do anything - cap off the line on the uncatted H. If you bypass the pump, you can cap the end of the line that comes from the pump. If you dont bypass the pump, route the end of the hose from the pump up high so it doesnt get crap in it. I would probably put a breather element on it to keep crud out, if you think you'll need the pump again.
I edited my post because I realized that I didnt know which component you wanted to deal with (the AIR pump's line to the cats, or the stupid little bung on the H-pipe). My original post was about the stupid little H-pipe fitting. Dont cap off the AIR pump line if you keep the AIR pump hooked up (the pump builds positive pressure and will blow a hose, etc)
Sorry for the confusion - I didnt think enough about what you might be asking yesterday. A lot of folks have an O/R pipe with that fitting for the smog pump hose, and dont want an exhaust leak, and that's what I thought you meant. I wasnt, and still am not, sure which you're askin about.
Well chit man, I overthought that one needlessly. WIth no cats, you can plug that little bung to the H-pipe.
Some guys crush it, and some have rigged up a cap from the parts store IIRC. If you wanna get this done right now, do a quick search. I haven't capped mine since I run a catted H at times and AIR pump at all times.