Sudden mileage drop puzzling??

[tradertcb]

On two occasions, my 05 Mountaineer with a V8, made a very loud bang on starting. The only thing I can compare it to would be an old fashioned backfire type noise. Now, mileage, at highway speeds, as indicated on the on board system, has dropped 3-4 MPG. Had it to the dealer and as usual, they found nothing wrong..

Any ideas? Could a backfire have blown an O2 sensor or some other sensor and now the computer is feeding it too much fuel based on bad data from some sensor?

If this is the case, shouldn't an error code appear??? It's still under warranty.

HELP

 

[aldive]

Have you checked the function of the O2 sensors?

 

[rancherlee]

My guess is that if it was a backfire of some sort then I would start checking the Intake components or you may have even cracked the plastic intake causing a Vacuum leak. Check the PCV valve, PCV hoses, Intake tube, and MAF sensor/housing for leaks.