Bad Gas Mileage
 

I have a 2004 f150 XLT 5.4 and I was getting 18mpg avg. about 2 years ago, and since then it has been heading downhill. Now I get around 12 mpg avg, huge drop and I'm not sure why?

Here is what I have done:
Gas Filter
Transmission oil change
Air Filter
Tire air pressure check (right on)
Spark Plugs (nightmare)

I figured it was the spark plugs since I have 80,000 miles on it, but that didnt help at all.

Any ideas of what could cause this much of a drop? Anyone else having similar problems? Looking for suggestions on what I should do, I would like to be getting around the 16-18mpg again.

Something strange does happen once in a while driving and I try to accelerate, my truck will make a loud noise (like i have straight pipes) and my rpms and speed will not increase unless i either hammer it or i let off and hit the gas again? Any ideas what this is? Maybe this is causing my truck to lose mpgs? This did start about 2 years ago?

Id take it to a exhaust shop and get them to check your Cat. o2 sensors may be getting weak also..

02 Sensors are consumable items... they should only last 60k-80k miles. Any longer, and you are on borrowed time.

I've got 50k on my truck, and I am going to replace them this year. It's too easy of a job to neglect, at least on my truck.

Just expensive because I got 4 I have to replace, and Tasca wants ~$220 for them all including shipping... but in the long run, it saves money on gas and pollution.

Probably damaged/plugged CAT(s) caused buy ethanol in fuel.

Ethanol isn't likely a cause of failed cats/sensors. Overly rich fuel mixture can be though, whether it's ethanol-laced or not.

O2 Sensors, clean throttle body, clean MAF, driving style. I would start with the cheapest/ easiest stuff first then work my way up. Winter blend and ethanol will also knock a couple MPGs off as well. Combine the winter blend and ethanol with a bad sensor or dirty throttle body and MAF sensor and 12 is in the ball park.

If it was a bad sensor, wouldnt my check engine light be on??

I think Im leaning more towards my cat. Seems to make sense since the loud noise sounds like straight pipes.....so maybe this is it??

I've seen a fuel injection flush give back 1-3 MPGs if its never been done before and you're over 90,000 miles.

Not necessarily - it could be getting "lazy" and still not bad enough to trip the CEL.