do you have a EGR ???

 

Could you have dripping injectors? This is caused by bad o-rings at the injectors. When it happend to me my gas mileage was 11 or 12 mpg.

 

Take a look at the throttle body bore, where the throttle blade lives. If it's gunked up, it's time for a cleaning that is best done with tb removed. Check the seal between the tb and manifold for damage, or better yet just replace it. Inspect the throttle shaft that the throttle blade is mounted on for wear at each end where it pivots in the tb. For whatever reason it sounds like its running pig rich. Any black smoke while driving or accelerating?