The PCV valve does exactly what it's name suggests, it provides "Positive Crankcase Ventilation". So if the pcv valve gets clogged then it won't provide proper crankcase ventilation which can cause numerous problems including: severe oil leaks, damaged gaskets, and excessive emissions.
Now that you know why you should change it at every "tune up", we're going to show you where it's located.
The PCV valve is actually located at the end of a hose, and is held in a hole in the lower intake manifold by a rubber grommet. Locating the PCV valve for the first time can be slightly difficult, but once its found it's easy to get to. You may have to lean up there with a flash light and look around a little to find it for the first time. It pulls easily out of the lower intake.
When PCV valve Is in the car the large end pushes into a rubber grommet in the lower intake manifold and a rubber hose attaches to the small side.
Right underneath the pcv valve (in the same hole) is a small mesh screen called a "spark arrestor" which filters out some of the bigger dirt and debris before it gets to the pcv valve. Many high mileage Mustangs still have the stock spark arrestor that's usually clogged which prevents proper pcv valve operation. It's definitely worth changing, and only cost $3-$4 at any Ford dealership. Below is a drawing of what the spark arrestor looks like.
Changing the spark arrestor can be much more difficult then changing the pcv valve. Although there are tools made specifically for changing this part there are other ways of going about it.
What we've found was easiest was to get a large self taping screw with a large head on it, and a pair of vise grips. Take the screw and screw it into the spark arrestor. Remember that the spark arrestor is pushed into a hole in the lower intake so it can be a very tight fit. Then take the vice grips and pull on the screw, which should pull the whole spark arrestor out. "If it first you don't succeed, try, try"..... well you get the idea. Once you get the spark arrestor out, all you have to do is push the new one in.
Be very careful not to drop the screw or anything else down the hole after you get the spark arrestor out.