In my 2004 Mercury Marauder, I was getting P0174 error code. I could not figure it out. Then lo and behold, I finally popped the hood and saw the PCV valve connector hose almost broken in half. I replaced the hose and what a difference!! It pays to sometimes just pop the hood and just look around. A simple $5 fix makes all the diff in the world.

https://www.youtube.com/watch?v=PThMYXXdB0Q