Have you gotten all the old gas out of the system? You say you have been out for awhile, and your dad drove the truck periodically. The question is, have you gotten ALL the old gas out? The new gas of today, doesn't like to be sitting for a while.
I would run the tank empty, drain anything left in the tank. Fill with fresh gas. Change out the fuel filter. And run some injector cleaner through. And see what that does. Because the new ethanol gas, draws alot of water. And seems to varnish up easier on rigs sitting.
This is where I would start.
