I have 150k on my 03 Civic it has overheated several times. I can drive the car around town without it overheating, but when I drive it on the highway for an hour and it will overheat. I thought I had a blown head gasket turns out I do not. I pulled the car apart and pulled off the cylinder head. I took it to a machine shop and they checked it for flatness I got an okay from the machinist. I was told that if it was a cracked head that one of the cylinders would be steam cleaned. Nothing of the sort here. I am now left perplexed, what else could cause an overheating issue. I've had the water pump, stat, rad and fans checked and replaced. Two master techs suspected a small crack in the head gasket. Should i get a second opinion on the cylinder head being cracked? How about a cracked cylinder wall or a cracked block? How might one check for such things, such as tests. I poured blue devil engine sealant in this car before and it was a huge mistake. When I pulled off the head there was what looked to be white rock salt all in the cooling jackets and coolant holes. I got a lot of that out but not sure how to get the rest that must be in the cooling system. I pulled a big chunk of the sealant that was blocking up the thermostat. Before the sealant was put in the car, the car could run for an hour on the highway before overheating after the sealant it would run for a few minutes and overheat. This is probably due to the thermostat being completely blocked off by sealant. Here are pictures of the head gasket, the cylinder head, and the block.