If your idle is weird you could have a leak in the intake boot. Unmetered air getting in after the MAF causes bad/unusual idle characteristics and dumps your fuel economy. The boots will develop cracks deep in the bellows and you have to REALLY look to find them. Smoke it (engine off obviously) before and after the throttle plate to be sure, or just a good inspection of the intake with it off the car to start with, may find it fast. Last option if there is no vac leak (which it sounds like there is) take it in and have the MAF checked out.