Positive Crankcase Ventilation
WARNING: This page is about a different car, the 1989 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.
Run engine at idle. Remove ventilation hose from PCV valve, hissing noise should be heard from hose and a strong vacuum should be felt immediately when finger is placed over valve inlet. If not, check hose and hose connections for leaks or obstructions. Check PCV valve. Service or replace as necessary.