what is this valve for?

http://www.mooimage.com/images/ubt5v3l6byuqu2xq99k_thumb.jpg (http://www.mooimage.com/viewer.php?file=ubt5v3l6byuqu2xq99k.jpg)

I noticed the tubing didn't fit at all so it must be getting false air. I assume it is for vacuum.... have replaced it with a fitting tube.
But what does this valve do and what wouldn't function right while I was driving with it?

The valve is on top of the engine (91 PA base).. it says "made in Germany 050"

The "valve" shown in the pic is your fuel pressure regulator. A few (of many) symptoms of a vacuum leak, would be unstable rpm & idle, hesitation & stumbling off idle throttle, mainly due to unmeasured air into the intake. In addition, and in this particular instance, the fuel pressure will be skewed due to lack of vacuum to the regulator

Thanks, I learned something again ! There is nothing in Haynes about this.:shakehead
I didn't have any of those symptoms, and drove the PA for over an hour today and everything was fine as usual...
Oh well. Now I'm certain the tube is a tight fit.