hi folks....
Your mileage is at the low end of the range I'd advise my customers to accept but there are many variables. Automatic trans....less mpg, A/C......less mpg, deferred maintenance....less mpg.
If you are truly running rich pull the dipstick and smell the oil. Oil contaminated with excess fuel will have an acrid odor. Excess fuel vapor from a rich condition will also throw off the O2 sensor, causing it to adjust too lean leading to incomplete combustion and lower fuel efficiency. Possibly change oil combined with the O2 sensor.
Mass air flow problem less likely especially if it runs reasonably well otherwise. EXCEPT, we purchased [once only] several reman MAFs from an otherwise reputable firm which had faulty ambient temperature circuits. They started and ran just fine but had poor mileage. Which leads to....
Coolant temperature sensor issues. Think choke. A CTS can degrade and convince the ECM that engine temp is 20 or 30 degrees less than actual. Just like a partially stuck choke will degrade mileage so too will a CTS out of range.
Should you take it to a dealer or specialist ? I'm biased, of course, but generally understanding the electronic systems and strategies are beyond the knowledge of an otherwise skilled home mechanic. I have numerous customers perfectly capable of doing brakes, timing belts, clutches, etc. who [wisely, in my estimation] put electrical/electronic management systems in other hands.
pat hayes preferred fleet medford, oregon