PCV Valve
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Infiniti QX4, 2003 Infiniti Q45, 2003 Infiniti M45, 2003 Infiniti I35, and 2003 Infiniti G35. However, it is still accessible from the selected car via links, so may be relevant.
With engine idling, remove PCV valve, leaving vacuum hose connected. A hissing noise should be heard as air passes through valve. A strong vacuum should be felt immediately when a finger is placed over valve inlet. If operation is not as specified, replace PCV valve.