I just bought an Ford F150 with an inline 6. I replaced all the freeze plugs. The previous owner replaced the water pump so that's also new. After getting the one at the bottom of the engine behind the transmission I replaced the one at the top in the back above it. I drove it a little ways and it blew the last one out. What would cause it to blow out? It was seated and snug so I don't think that it was out any. The truck had sat for quite a while and not driven before I bought it.

 

Replace the radiator cap. It may not be relieving excess pressure.