What would happen if the valve cover breather nipple was blocked? Like what if the hose going to it was pinched shut, or if I just put a cap on it? I don't plan on doing it but I had some problems with my car last year and over the winter I found the hose going to the valve breather was pinched shut.

 

a little rougher idle possibly?

 

would build up alot of pressure in the valve cover and eventually cause your valve cover gasket to leak oil. Under pressure it will coat your engine bay. I speak from experience from a clogged PCV valve (sucks believe me!). Replace the hose or get a breather filter and put that on. The purpose of the hose is to recirculate the gasses into the intake to be burned in the engine. Hope this helps.