Alfa Romeo Repair: electronic ignition system, electronic ignition system, altitude changes

Question
I have a 1989 (1988?) spider veloce.  I suddenly started to get a very high-pitched whistle coming from somewhere on the right side of the vehicle.  It would start as soon as the car was started and persist until I accelerated rapidly or put a load on the engine on a hill.  It returned as soon as I backed off the acceraltor.  After a lot searching with a rubber hose in my ear, I found the source under the panel on the right side of the car.  It was a device next to the electronic ignition computer.  It has a vacuum hose connected to one side, an open port in the other and an electrical connection on the side.  When I covere the port, the whistle would disappear.  It appears to have a diaphragm of some kind that is probably split.

I found a picture that looks like the device in the owners manual and it is called a "vacuum sensor".  In the Rayce spider catalog, there is a picture that looks like it and it is called an"auxilary air valve". I did disconect the vacuum line and plugged it with a small bolt.  The idle rose by about 50 rpm, but I can't detect any difference in performance.  The car is mostly driven along the coast here in CA on weekends, so it does not see any significant altitude changes.

What does this thing do, what is it exactly, and do I need to replce it?  If it is the device I found in the catalogs the price varies between $179 and $299!  Thanks

Answer
It is correctly called a pressure sensor--it is an electronic vacuum advance for the ignition timing. For your purpose it is not a critical item--if you did a lot of highway cruising this device could add 2-3 mpg.  The auxillery air valve has a hose on both ends and is a high idle device for cold starts, it is on the valve cover of the engine.

So, block off the vacuum hose and be done with it.