Recent drop in MPG (17 MPG to 13 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.
A year later (now) I 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 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 live data app on my OBD11 reader, 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 a 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

 

[99WhiteC5Coupe]

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.
A year later (now) I 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 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 live data app on my OBD11 reader, 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 a 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

I have found that before oxygen sensors fail and set a trouble code, they get “lazy” and fuel economy drops.

240,000 miles on the original oxygen sensors is amazing and almost unheard of. I cannot imagine that they are operating at peak efficiency or data reporting?

My only suggestion would be to replace both sensors, using the OEM Motorcraft brand.

 

[Gary Waugh]

I have found that before oxygen sensors fail and set a trouble code, they get “lazy” and fuel economy drops.

240,000 miles on the original oxygen sensors is amazing and almost unheard of. I cannot imagine that they are operating at peak efficiency or data reporting?

My only suggestion would be to replace both sensors, using the OEM Motorcraft brand.

When you say both sensors, do you mean both sensors on each engine bank? There is a sensors before the catalytic converter and a sensor after the cat (2 per side, 4 sensors total) on each bank of cylinders? So change all 4 sensors, or did you mean 2 (both) sensors, which 2 are you referring too?

Regard Gary

 

[Fastcar]

When you say both sensors, do you mean the 2 sensors on each side of the engine? There is a sensors before the catalytic converter and a sensor after the cat (2 per side, 4 sensors total) on each bank of cylinders?

Regard Gary

When you did the test before this one what was the temperature outside?

 

[Gary Waugh]

It was probably about 20 F warmer than now. It was 58F today and was in the high 70’s last time I tried this test, the engine was up to normal operating temperature every time I tried the test.

 

[Fastcar]

It was probably about 20 F warmer than now. It was 58F today and was in the high 70’s last time I tried this test, the engine was up to normal operating temperature every time I tried the test.

Have you checked your tire pressure?

 

[Gary Waugh]

Good question, and yes all tire pressures were checked (just routine for me) so I didn’t check them specifically for this test, but I check them every week (checked them on Monday).

 

[Fastcar]

Good question, and yes all tire pressures were checked (just routine for me) so I didn’t check them specifically for this test, but I check them every week (checked them on Monday).

A couple of more ?'s. Do you folks use winter gas? Did your tire pressure drop much from from the higher pressure to the 58 degrees? I'm asking 'cause tire pressure here in FL can vary by 5+ lbs from side to side when parked one side in the sun and the other in shade

 

[99WhiteC5Coupe]

When you say both sensors, do you mean both sensors on each engine bank? There is a sensors before the catalytic converter and a sensor after the cat (2 per side, 4 sensors total) on each bank of cylinders? So change all 4 sensors, or if you do mean 2 (both) sensors, which 2 are you referring too?

Regard Gary

I meant all 4 sensors - before and after the catalytic converters. (I meant to say both sets).

 

[Gary Waugh]

the tyre pressure changes quite significantly with the temperature change, i regularly see a 5-8 psi change in pressure when the temp changes by 20-30F between our Texas summer and winter.

 

[Fastcar]

the tyre pressure changes quite significantly with the temperature change, i regularly see a 5-8 psi change in pressure when the temp changes by 20-30F between our Texas summer and winter.

Do you folks use winter gas? I ask cause living in MA it would lower mileage by 2-3 mpg of course that was 20 years ago. I'm thinking that low tire pressure and winter gas would account for the drop in mileage.

 

[Gary Waugh]

I am not aware of any switch in gas between the summer and winter, but I will definitely do some searching and see what I find. It’s not something the gas stations advertise, but that doesn’t mean they don’t change the gas for the seasons.
Thanks for the suggestion.

 

[razrback]

Just a dumb question, but have you actually noticed the drop in MPG? Since I have no idea how the car calculates MPG, just wondering if the calc is off, but actual is not.

 

[Flight-ER-Doc]

240k on the original spark plugs?

 

[Burt1631]

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.
A year later (now) I 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 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 live data app on my OBD11 reader, 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 a 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

I would check the catalytic converters....have you heard a sound like small rocks in a can? Tap on the converters and listen for a rattle sound.

 

[Limoguy25]

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.
A year later (now) I 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 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 live data app on my OBD11 reader, 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 a 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

Is this 4x4? If so, check to see that both front locking hubs disengage when the engine is running.

 

[rlhaberm]

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.
A year later (now) I 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 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 live data app on my OBD11 reader, 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 a 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 did the exact same thing about 2 years ago @ 100K miles. I tried everything you tried, plus a “tune-up” by a Ford dealer that included new spark plugs…all with no appreciable improvement. My dealer started talking about maybe it’s time to replace the turbos, but I wasn’t convinced. About a year ago I was in Walmart, saw a display of Lucas fuel treatment, and thought “what the heck”. So i dropped $9, and within 2 days my mileage was back up to 18. Now I add it every other fueling, and mileage is still around 18. I’m not normally an “additive” kind of guy, but it’s working for me. IMO it’s worth a shot!

 

[Gary Waugh]

Thanks everyone, I have confirmed that the front wheel hubs do disengage when in 2H mode and 4A modes. I have regularly run Lucas fuel treatment and have been trying it for the last few hundred miles.
I have not heard any noise from the catalytic converters, but I did climb under the car and went around taping the converters, but couldn’t hear anything rattling inside. Still suspect that the O2 sensors might be the cause given the age and mileage they have done, but no idea how you test an O2 sensor and they are too expensive to just go and change out in the hope it helps (but I might still do that if I don’t find any other ideas).

 

[Fastcar]

Did you ever do a re-balance at a competent tire shop not at the dealer??

 

[Gary Waugh]

I had all 4 tyres replaced about 6 months ago and they did a balance as part of the tyre change. Have used this garage for 6+ years and they let me watch. They have always been very competent and I trust them completely for anything to do with wheels and tyres. The car has no shaking at any speed on the road.