I have recently bought a used 2001 F-150 4.6 the other day the engine light came on and it started to miss. I looked under the hood and found the top of a spark plug blown off and the plasitic bracket that screws the coil down was broken. II have replaced them but am worried this may happen again. Does anybody have an idea what may have caused this??

 

Which cylinder was it? These trucks are known to blow out plugs, some cylinders are more common than others, but can be due to thread wear, or over tightening are most of the reason behind it.

 

The plug either backed out from not being tightened enough or the threads in the head let go.
Cylinder combustion is very powerful and usually wrecks something in the process when this happens.
Good luck.

 

the thread did not strip or blow. what happened was the porcealin blew off the top whaile driving. it was the cylinder on the drivers side closest to the rad.