I have a 1988 F250 4x4 Lariat. 39,000 original miles on the 7.3L engine. Was stored for 15 years after my dad passed on. I proudly inherited it. After having a new front tank installed due to fuel issues, I have been driving it local to work a couple times each week. Recently I’m seeing blue, not black smoke out of the exhaust while driving. Sometimes a steady trail, other times not as much. And sometimes nothing at all. Any suggestions on what to look at?