The Mustang PCV Valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that's siphoned from the crankcase into the Mustang Intake Manifold. This is necessary because air drawn through the PCV Valve from the crankcase has a leaning effect on the fuel mixture much the same as a vacuum leak. So air flow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blowby is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase. It's important to note that Mustang PCV Valves are sized for specific engine applications. The wrong Mustang PCV Valve for an application can flow too much or too little air causing driveability problems. Varnish deposits can clog the valve, so replacement for preventative maintenance is recommended (every 50,000 miles usually). This also goes for the 86-93 Mustang PCV Valve Filter Screen. We carry the best PCV Valves for your Mustang from Ford Motorcraft.