what is it where is it and how do I replace it? why do I need to replace it at tune up also? thanks!

 

I dont have a picture but it is fairly easy. It is located very close to the accelerator/cruise cable mounts. You can actually see a small part of it because half is plugged into the engine cover and the other half is plugged into a black rubber hose and clamped. Remove the clamp and the hose. The valve comes out like a bottle cork.
The way I know this valve can stick closed after a while (from oil rezidue) and wont let the engine "breathe" or stays always open.