02 5.4L Eddie Bauer expedition recently changed oil and filter, fuel filter, air filter, ran sea foam through intake and gas tank, gas mileage increased from 9.6 mpg to 15.5 mpg after a week it has went back down to 11.9 mpg and is sluggish on take off at times and and doesn’t have the acceleration it should. Vehicle currently has 155k miles on it and I’m about to change the transmission filter add exhaust and a nice intake. Any info on why it could drop back down like that would be great. I love this truck and want it to stay around for a long time