Need some help. 2003 f 350 6.0 automatic.58,000 miles.All the factory updates,never chiped,regular filter and oil changes as recomended. On a 220 mile trip pulling about 10,000 pounds noticed a foul smell in the cab about half way. Smell went away. Inspection revealed low coolant.Toped off coolant Started engine and had steam out of the tail pipe.Could this bee egr valve or head gasket. Engine runs fine,no lose of power. Cracked the top on the degas bottle steam went away. What do you think?