Easy. If the manifold is leaking outside air into the exhaust system before the oxygen (02) sensor, the O2 sensor will think the truck is running lean and dump more fuel into that side of the engine. Hence worst fuel economy.

If you would like to test the theory out, feel free to loosen up the front O2 sensors on your D.

 

lol... good answer (grasping at straws now) but I still believe they actually did change the timing chain assembly. They also changed the front crank seal because it was leaking oil.

 

07 4.7 D. had the same ticking/ knocking noise. I put a can of Sea Foam in the tank and filled it up. Used another one from the hose off the brake booster. Waited 30 minutes. Blew it out. It has been 5 months and still no more knocking or ticking. I use Lucas injector cleaner every few tankfuls and basically was using Mobil 1 gas. Now that I am in Arkansas I cannot find Mobil 1. Does anyone have any ideas who the second best fuel to use for my D would be? Honestly I just see Shell, Conoco, Phillips, and the regular cheapees lke Km and Go, Walmart and whatever. There are a few Pilots and Flying J's. Any ideas??