My 2000 Sierra had a hairline crack in head that just steamed the water inside valve cover & congealed the oil into a thick jelly all caked up on top of valve cover. Never caused any problems besides slowly losing oil/coolant but I didnt let it go very long & replaced heads. That was 100k miles ago & its still running strong at 370k miles. I still drive it like I stole it but worry a lil when hitting 110+mph....