Poor fuel economy on a 2002 toyota tacoma 4 cyl

Mightyhunter, when was the last time you changed your oil? There was one time that I changed my oil that I'm pretty sure the stuff I bought (I think it was Valvoline from Advance Auto) was actually mislabeled. According to how my truck ran with it, the viscosity seemed like it was completely not 5w30. I had gone from getting ~25mpg on my '01 PreRunner 2.7l (I've messed with a lot on my intake and exhaust and I have an o2 simulator) down to about 15 mpg. After I changed the oil again, my mpg went back up to normal.

 

Like mentioned, a good ol' tune up and once over on all the sensors and vacuum lines to start. If that don't help maybe do some more in depth diagnostics like compression test just to be safe and rule out things like bad valve adjustments and such. Somethin is goin on as I pull 21-23mpg around town with my '97 5spd 2.7 4x4 with 300k on the clock.