Reduction in MPG

[Gary Waugh]

Hi everyone, I own an 08 Ford Expedition, have owned it from new, it has 240+K miles and nearly everything is still original on it. About a year ago (maybe a bit longer), I reset the fuel economy monitor (this car does not have an instant MPG, only an average MPG reading) and carefully drove the car about 30 miles, it averaged 17 MPG. I did this several times over a few months and consistently saw 17MPG. I recently did the same thing and I now have a real struggle to get anything better than 13 MPG. There are no check engine lights and when I scan codes with my OBD11 (mark 3) scanner, no errors appear. I recently wondered if maybe the oxygen sensors were going bad after 18 years and could be causing the higher fuel consumption (the engine does not burn any oil and idles and runs very smoothly). I just tried the OBDeleven live data app, unfortunately this doesn't show much data for my car, but does allow me to see short and long term fuel trim values. The short term fuel trim value seems to vary from -10% to +10%, from my understanding this sounds very reasonable, after 30+ miles the long term fuel trim shows +2.6%. Can anyone confirm is these values are reasonable and if they would give any indication of a poor oxygen sensor? Also does anyone have any idea what could change to drop the MPG from 17 to 13? I have cleaned the MAF sensor (using MAF cleaner), I replaced all 8 spark plugs (the old plugs only had 30K miles on them and look great), the air filter was replaced (again the old filter was not very dirty), the engine oil has been replaced every 5K miles with Mobil 1 synthetic, the transmission and differential (+ transfer box) oils have been replaced every 70K miles. I fitted a new Motorcraft fuel filter, but still only see 13 MPG! The car runs great but I would like to get it back to the earlier 17 MPG if I can, I am running out of ideas on what could be wrong, hence my clutching at the oxygen sensor straw!! If the sensors had failed I would expect an OBD code, but maybe they are getting old/weak and should be replaced? Would blocked catalytic converters cause a poor MPG without giving an error code (I would expect that if the difference between the O2 sensor before and after the cat was too small/large, then the car would report an error?), However O2 sensors and replacement catalytics are expensive and difficult to access, so I want a bit more proof that they should be changed before I dig into it.. Thanks for and any suggestions. Gary

 

[AWAR]

Have they increased the percentage of Ethanol in your fuel? Maybe a Summer blend on the fuel.

 

[Gary Waugh]

I guess that’s possible, but would it really impact MPG by 20%? Don’t know how I can check if it’s that, I don’t have any garages that sell ethanol free gas within 40 miles of me to see if the MPG goes up using ethanol free.

 

[99WhiteC5Coupe]

I have seen vehicles that have the mileage drop and had one or more defective brake components causing brake drag (sticking caliper, sticking brake pads, internally collapsed brake hose, etc.).

After a long drive, perhaps use your hand to feel each of the wheels to see if one is hotter than the rest (or use a handheld infrared thermometer)?

I have also replaced oxygen sensors that were somewhat high mileage (100,000) - but not 240,000, that have improved the fuel mileage. I have had trusted mechanics tell me that oxygen sensors get weak or “lazy”, before they set a trouble code - and can cause a drop in fuel mileage.

 

[AWAR]

Did you reset the trip meter as well as the gas mileage meter. I think they average for total trip meter, the newer trucks do.

 

[GlennSullivan]

Above suggestions for O2 sensors and partially plugged CAT make sense.

I would add that you also do a manual fuel mile calculation. I.E. fill vehicle, note mileage, drive most of the tank, refill vehicle, note gallons and miles and do the math. Over the years I’ve seen the mileage on our various Expys not report well.

FYI
Ethanol Percentage (E10 vs E15): Standard TX pump fuel remains E10 (10% ethanol) in both seasons for most vehicles. However, emergency or seasonal federal exceptions can introduce or extend E15 (15% ethanol) availability into summer months to help manage fuel pricing and supplies.

 

[Gary Waugh]

Thanks everyone, I have been noting the mileage at every fill up and the amount of fuel I add, my calculated mpg and the displayed mpg match to within .3mpg every time. I did raise the car yesterday after a 50 mile trip and tried rotating the wheels, there was no noticeable drag and all the brakes felt to be equally warm, so I don’t think any brakes are dragging.. really suspect some sensors are just getting old and probably need changing. Need to look at the plugs and see if they show signs of running rich.