The PCV (Positive Crankcase Ventilation) valve is part of an emissions system designed to divert gasses than escape past the piston rings back to the air intake to be burned by the engine.
I will describe how to check and replace a PCV valve using my 1996 Ford Ranger as an example.
1. Locate the PCV valve. It will usually be on a valve cover or somewhere around the top of the engine.
2. Disconnect the hose from the valve.
3. Pull the valve out and shake it. It should rattle. If it doesn't rattle, the valve is plugged and needs to be replaced. Also check the hose by disconnecting it and blowing through it to check to see if it is clogged.
4. Installation is the reverse of these steps.
Replacing the PCV valve is an inexpensive maintenance item you should consider doing every few years. In the case of the Ranger with the 4.0L engine, it is a ~$2 and five minute preventative maintenance job that can keep your engine and emissions system in top shape.