My 03 EB just doesn't get the fuel mileage it use to. If I cruised at 70mph it would get 17.1mpg with 3 -4 people in the vehicle. on 87 octane. This was around 50,000-70,000 miles on the Expedition. Same tires, same everything as right now. At 50,000 miles the timing set was replaced but still maintained the same fuel mileage prior to that. I have stayed up on maintenance Change oil every 3,000 miles with motorcraft oil. Recently replaced the plugs and coils with OEM. Fuel filter is new. Cleaned the MAF sensor although it wasn't dirty. It has had a K&N air filter for quite some time and I keep it clean with fresh oil on it. Now its doing good to be getting 13.1 on the highway with minimal city driving. The last couple years I have been running 89 octane in it most of the time. I recently took it from central Missouri to South Georgia going through Alabama. I pulled a 54"x10' long trailer. I ran 87 octane and got it to 13.4mpg all highway. I picked up 4 tires and wheels that weighed about 800lbs. On the way back I started running 89 octane and the fuel mileage came up to 13.7mpg while pulling more weight. The thing runs flawless and smooth. At idle you cant even tell its running unless you look at the tach. The tail pipe is just as clean on the inside as it is on the outside. What am I missing or what can I do to get it back up closer to 17 on the highway and 13 around town? It has just hit 98,000 miles and I drive this thing extremely easy. Slow gentle take offs and run the speed limit or 5 under.

 

In the summer weather I can get 16-17 MPGs with my 2006 Navigator running ~70 MPH. As soon as the fall weather hits and the fuel blend changes it drops to 13.5 not matter how I drive. I usually run 87 octane, tried 89 and 91 with no alcohol but no change. Frustrating for sure.

 

I put K&N CAI and 78mm throttle body, I porting throttle elbow, I have 285/65/17 tires and... my 4,6 takes 11,5 MPG

 

Normal wear and tear is a factor you don't seem to have mentioned. The valves, cam lobes, rings, cylinder walls all wear as time goes on and as a result the compression starts to fall which decreases fuel efficiency and MPG.

 

This Expedition along with 3 other vehicles I have I’ve owned and still own for the last 200,000 miles, their fuel mileage hasn’t dropped this much as my Expedition has in the last 40,000 miles and is still under 100,000 miles. I would think if it’s lost that much compression I would notice an oil consumption problem or power loss. Same goes for cam lobes. It runs smooth and still has good power. When i put plugs and coils in it within the last 10,000 miles, all the plugs come out clean as in no build up of oil or carbon. As I did mention this Expedition gets pampered and driven gentle. It’s only had 3 trailers hooked to it it’s whole life. The most one has ever weighed was less than 2,000lbs. The third seat has been folded up and sat in one time it’s whole life.

 

It would only burn more oil if the rings were worn, if the loss of compression was due to burnt valves no additional oil would be burnt. As to no loss of power, that's hard to judge because you could simply be applying more gas pedal to compensate without ever realizing it. The only way to know for sure would have been to measure the amount of pedal travel then and now to get the required power and see if there was a difference in required pedal travel.

 

Does it have any codes stored?

Is the intake tract coated in residue from the K&N filter? The residue will build up and mess up sensors over time if the filter is over-oiled, of even if perfectly oiled, some still gets through.

Is the converter clutch engaging properly?

 

I'm think that it is K&N. I had the same after changed stock air filter to K&N CAI. More air you have more gasoline you take.

Try to put stock filter and compare it.

 

Don't forget the "quality" of fuel matter most! When home in Ohio my 2003 Expedition when keeping it under 75 on the highway I would average around 14.7 to 15 on the highway, take it to Iowa/Wisconsin and it would drop to 14 to 14.2. With nothing changing other than fuel(they use a lot more ethenol).

My 1997 expedition got a best of 14.82 MPG ever, and my 2003 expedition got a best of 15.2 MPG ever. I can't imagine buying one of these and expecting any better. Don't go by the lie-o-meter. Its NEVER right! Always hand calculate it.

Ironically I bought a 6.0 diesel excursion and it gets better mileage than the expedition did.

 

I will probably catch some hell from some of you, but the throttle body is what controls how much air goes into the engine. I have never seen anyone ever getting better mileage off just changing a stock filter to a K&N, In fact it probably gets worse. Factory air box takes air from the inside fender area which is cooler than what's under the hood... K&N gets it from under the hood.

 

Premium gas helps... i got 4.6 for yrs...i rented a 2019 expy mines 03. Both from newark nj airport to daytona fl airport both right off 95. Gets there on 3rd tank with over a 1/4 tank. 700 miles

 

It's my average fuel milage on premium gasoline 98 octane ( in europe 95 octane is standard )