So, we went wheeling today and a couple of the guys with me think the truck is running poorly. They said it is visibly smoking as well as "sounds" like it is running on 5 cylinders. I'm arguing that if it had such a bad misfire, that it would throw a code and the check engine light would come on.
I'm wondering if the catalytic converter is clogging up due to the bad valve stem seals?

On occasion it feels like it is lugging pretty bad and I do get a bit of a shudder at 40mph-ish.