Valve Springs
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2001 Ford Pickup, 2001 Ford Expedition, 2001 Ford Econoline, 2000 Ford Pickup, and 2000 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.
Inspect valve spring free length, out-of-square and pressure. Replace valve spring if not as specified. See VALVES & VALVE SPRINGS table under ENGINE SPECIFICATIONS.