The purpose of the positive crankcase ventilation valve - or PCV valve - is to recirculate exhaust gases that blow into the crankcase back into the engine for more complete burning. It also reduces emissions. After roughly 100,000 miles, you may need to replace the PCV valve.
Locate the PCV valve, which is located on the driver's side of the engine block on a Ford Focus and looks like a plug or stopper. The exact location varies with the model. On the base model, it's located on the back side of the valve cover. On the ZX3 model, however, look to the right of the oil filler cap and then eight inches down on the side of the block.
Note the orientation of the valve, and then pull the connecting hose off the end of the valve. It should pull straight off.
Grab the end of the PCV valve with the pliers and twist the valve counterclockwise. You may have to twist fairly hard to remove it, but there is nothing else holding the valve in place.
Install the new PCV valve with the same orientation as the old valve. The valve simply "plugs" into the hole where the old one was located.
Attach the hose to the end of the PCV valve.
Start the engine and wait for the engine to heat up. When the engine is at normal operating temperature, place a rag over the hose connected to the PCV valve and gently squeeze the hose with a pair of pliers. If you hear a clicking sound when the hose is pinched off, then the valve is operating correctly.