Just thought I would pass along an interesting problem I ran across with the PCV system on our 95 F150, 300cid, 5speed truck. When we performed a key on running test the computer gave us a code that indicated it could not control high idle. This in fact was confirmed because the truck when warm idled higher than normal...around 1300 rpm....the computer was unable to bring the idle down to 750 - 800 rpm mark. After checking the idle air control system...it turns out the PCV valve was allowing to much vaccum to pass through. This valve is a metered device and can either not allow vaccum through causing engine sluge to build up or stick open and not meter the flow causing a high idle condition the computer can not compensate.

You can duplicate this effect by just pulling a vaccum line off of the manifold and let it suck air.....the engine will increase idle speed.

Hope this helps someone......if you replace the valve and it still idles high...it may indicate a vaccum leak somewhere else...provided the idle control system is working properly and someone did not monkey around with the idle stop screw. I have learned from experience these engines are extremely prone to vaccum leaks....we recently replace the upper intake gasket and the thorttle body gasket....this helped the drivability tremendously and the truck is getting much better fuel economy. It did not seem ford used very good materials in the gaskets when the truck was made.