Whats the pcv valve, and where is it?

PCV stands for positive crankcase ventilation. Pressure builds in the crankcase because no ring seal is perfect - so some cylinder pressure gets by the rings and into the crankcase. This pressure has to be vented in some way. Cars used to just have draft tubes that stuck down under the car -- you've probably seen older cars with a bit of oil smoke coming out of those tubes (sometimes lots of oil smoke). Environmental regs ultimately required that venting to be passed back through the engine to clean up the air. The PCV valve is simply a check valve that allows crankcase pressure to leave the crankcase and enter the intake manifold - whenever there is vacuum present in the intake. On our 5.0L's it's located in the back of the lower intake manifold. Very hard to see and get to. The egr valve/10pins get in the way on the passenger side, the upper plenum gets in the way on the driver's side. But in the center of the back of the lower intake, there is a rubber grommet. The pcv sticks into that grommet, and a vacuum hose attaches to it and runs to an upper intake vacuum port. Below the grommet and valve is a metal screen/oil trap that helps to keep oil from entering the intake. If you're gonna freshen up your pcv system and it hasn't been touched in 100K - you should go to your local Ford dealer and get a new metal screen, grommet and pcv valve (motorcraft brand). It'll cost about $18 for all of it. I'd also replace the rubber hose as it will probably be quite brittle. Pull the pcv out, remove/replace the hose. Remove the rubber grommet from the intake. Use a wand/magnet to pull the metal screen out. Replace with the new one, put the new grommet in and put the new pcv back in the grommet, reconnect the vacuum hose. I'd stay away from cheap $2 pcv valves - they usually don't fit tight enough in the grommet (especially an older 'cooked' grommet) and cause vacuum leaks. The other hose to check is the one that goes from the valve cover oil filler neck nipple to the throttle body. Air in that hose is bi-directional. When vacuum is pulling on the intake through the pcv valve, air has to enter the engine from somewhere. It comes in through that hose after it's been metered by the maf. Under full throttle conditions when there is no vacuum and crankcase pressures are higher, crankcase pressure vents into the intake manifold through the pcv, and it also vents through the valve cover hose into the throttle body.

When you buy your new pcv valve, get the size that matches it. It's probably 3/8th vacuum hose. Buy a foot or so and cut to match.