The ejected spark plug reamed out the spark plug threads in the head, making a Helicoil repair impossible. A re manufactured head (the engine is a V6, so it has two heads) is $700, but the major cost is about 20 hours' worth of labor. Apparently the defective design of Ford's aluminum heads (too few threads?) allows the spark plugs to, over many miles, unscrew themselves and get exploded out of the head. There are many MANY reports of this unexpected behavior. When the mechanic went to start up the Tribute after it was towed to the garage, he noticed flames shooting out of the engine. We were VERY lucky that the gasoline being injected out the spark plug hole did not catch file from the loose coil wiring, resulting in a disastrous under-hood fire. This is the first car my daughter ever bought with her own money. Now she knows the "joys" of buying a used car.