Op, you might want to read this thread
Link where someone who owned one of these engines pulled the heads off at 70K and see what they found.
He mechanically cleaned up his valves and installed a catch can. The reason for the valve carbon problems is the PCV intake line is placed in the intake tract. If you live in a warmer climate the oil gets fumie and is sucked into the intake tract where it can carbon up on the back side of the valves because they are very hot.
The catch can idea keeps the fumes out of the intake tract and keeps the valves clean. It's a pretty good solution to the problem.
Ford saw the light and fixed this problem once and for all with their port injection. :)
