I have a `92 ford ranger that has had frequent cooling system problems. I have had to replace 4 of the 6 frost plugs, and after the most recent replacement, it has not ran the same. It has difficulty starting and idling. I have noticed water in the oil, as well as white smoke coming out of the tailpipe, and a general lack of performance. Is it a cracked head, warped head, blown head gasket, or have I cracked a cylinder.Also how can I tell if the head is machinable, or if it is damaged upon all repair. If it is damaged, what is the cost of replacing a head?