On the Zetec, the PCV valve is located in the front of the engine,
behind the lower right corner (as you face the engine) of the aluminum
exhaust heat shield. It is in a rubber grommet, with a hose attached -
a bit hard to reach. Do this with the engine cold, so you don't burn
your fingers. I typically clean mine with carb cleaner and reinstall.
It is always clean and the plunger inside is not stuck.