Reduction in MPG

Gary Waugh · Saturday at 12:58 PM

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 · Saturday at 2:33 PM

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

Gary Waugh · Saturday at 7:07 PM

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 · Saturday at 8:18 PM

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 · Saturday at 10:25 PM

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 · Sunday at 9:56 AM

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 · Sunday at 10:53 AM

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.

Glen B Ashcroft · 2026-07-27T12:00:27-0400

we experienced this with a bad fuel pump, but also got fuel/air mixture computer code, dropped fuel pump at 180K

JayB66 · 2026-07-27T13:22:49-0400

When did you last check your filters? A partially clogged air or fuel filter is your least expensive suspect. And if you managed to get some crappy gas somewhere....

rlhaberm · 2026-07-27T14:24:39-0400

Gary Waugh said:
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

My 2017 has been doing this this on a fairly regular basis since about 50K miles. Multiple dealers have tested & checked sensors, had a tune up and plugs replaced at 90K, and still happens. At one point I thought maybe they replaced the oil with heavier viscosity & had them replace it again with the recommended 5W 30 synthetic. Seemed to help for a while, then randomly the mileage dropped again. Happened again last week while traveling from Charlotte to Colorado. Out of desperation a few years ago I added a bottle of Lucas fuel treatment to a fresh tank of gas, and almost immediately saw it improve from 13ish back to 18. I normally add some every 3-4 tanks, but hadn’t in quite a while. I put a bottle in while in Hannibal, MO, and by the time we reached St. Joseph was back to 18 MPG. I’ve never been a believer in additives, but this seems to work. Might be worth the $10 to see if it helps your situation.

Gary Waugh · 2026-07-27T15:57:22-0400

Thanks for the suggestions, I have and still run Lucas fuel treatment so I can’t see that helping, I add 6 oz to every fill up. I changed the air filter about 400 miles ago, genuine motorcraft part, but it made no difference. I will get a fuel pressure gauge and check the line pressure, i did fit a new filter about 400 miles ago (when I changed the air filter, oil filter and engine oil).

Limoguy25 · 2026-07-27T16:54:31-0400

Gary Waugh said:
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

Gary, is this a 4x4?

rslynes · 2026-07-27T16:58:26-0400

Dirty injectors will make MPG drop a bit too as the fuel 'fog' is inconsistent and doesn't burn efficiently. I prefer Techron Concentrate Plus, Redline SI-1, or any good fuel system additive with a decent amount of polyether amine (PFA), and I'll use a 20oz. bottle of that with a full tank every 2,500-3,000 miles.

I have over 180K miles on the clock and still get anywhere from 17 mpg at 75mph to 19 mpg at 60mph (unless it's all uphill or driving into a strong headwind). I do change plugs every 50-60k miles, too.

Gary Waugh · 2026-07-27T18:43:28-0400

I will give the Techron a try (nothing to lose). The car is a 4x4.

redoldsman · 2026-07-27T20:01:30-0400

I have a 2012 2WD EL with 150,000 miles on it. I had the timing chains and sensors replaced at around 125,000 miles. Last year I replace all the plugs and coil packs. Both converters and all the sensors have been replaced. I bought it with approximately 77,000 miles on it. It has always averaged about 13MPG around town and the best I ever got on the highway was about 15MPG. I had a 99 years ago and it would average a little over 17 on the highway. I see you are in Princeton. You can get ethanol free gas at Buc-E's.

GT350driver · 2026-07-27T20:20:53-0400

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

I just heard yesterday from the owner of local speed shop who I just stopped in on Saturday to ask the same question. I installed a K&N filter about 3 months ago and my 18.1 mpg progressively dropped to 14 mpg. I thought it was a contaminated MAF sensor but just found out the Ecoboost twin turbo doesn't have one. He told me the 87 blend has been showing an increase in ethanol percentage.
He also said I shouldn't be running anything less than 89 octane ..... 2 days after I filled the tank

mr_dave · 2026-07-27T21:02:35-0400

What about stuck or partially stuck piston rings? With all of those miles it's a definite possibility.

I'd highly recommend the new Valvoline Restore and Protect oil, which is proven to remove deposits. I've been using it for a while and mine runs like new.

Fastcar · 2026-07-27T21:11:12-0400

mr_dave said:
What about stuck or partially stuck piston rings? With all of those miles it's a definite possibility.

I'd highly recommend the new Valvoline Restore and Protect oil, which is proven to remove deposits. I've been using it for a while and mine runs like new.

Another mechanic in a can! If it is removing deposits where are the deposits going? Maybe mixing with the oil and clogging oil passages? Maybe the oil pump? What if it unseats the piston rings? Stay away from this stuff if you have a bunch of miles on it.

mr_dave · 2026-07-27T21:21:47-0400

Fastcar said:
Another mechanic in a can! If it is removing deposits where are the deposits going? Maybe mixing with the oil and clogging oil passages? Maybe the oil pump? What if it unseats the piston rings? Stay away from this stuff if you have a bunch of miles on it.

I'd recommend doing some research before stating outdated opinions with such confidence.

Here let me help get you started: https://youtube.com/shorts/xRglHB_0nTU

Gary Waugh · 2026-07-27T21:40:25-0400

The car has run on Mobil1 synthetic all its life (I bought it new in 08), compression on all 8 cylinders showed 145 PSI + or - 10 psi, so I really doubt the rings are stuck or excessively worn. The oil has been changed every 3000 miles (I always thought this overkill, but it’s what was recommended many years ago and I stuck with it). Still believe it’s running rich (stuck injectors, fuel pressure too high, bad oxygen sensor, etc) but need to check more.
Thanks for all the suggestions.

Reduction in MPG

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