Hi, I have an 06 Titan with 154k on it. A month or so ago I was having some rough idle and hesitation with jittery acceleration so I put new plugs in and seemed to be running a lot better. Today my in laws barrowed the truck and it started running really rough. Then a loss of power like it's hard to get to 40 mph. And then we noticed it was starting to smoke under the hood. Smells like exhaust. Couldn't see a leak or anything. It never came close to over heating or anything. Am I looking at a cracked header? Or any other suggestions would be greatly appreciated.