consistently worse MPG recently

[rjl2001]

I have a 2000 DII with 125,000 miles. Have kept track of MPG, and was always right around 300 miles per tank (~22gals). For the last few months, I have consistently been getting 250 miles per tank. I didn't do any modifications or changes during the time it went down. I have had issues with the CEL pretty much since I've owned the truck, but has not been on since this has been going on.

Any ideas on what sort of things I should be looking at as possible causes?

 

[achampagne]

maybe its time for a tune up? check your tire pressure?

 

[bmohan55]

just changed my plugs & wires and resulted in an average 2 mpg increase

 

[brianhoberg]

oil, air filter, fuel injector cleaner (try Seafoam) in the gas tank, plugs, wires. Consider changing your O2 sensors if they are older.

 

[DeadHead86]

Tune up. I'd go with 8mm wires. As mentioned above, Seafoam is great. I run it through my DI just about every time I change my oil.

 

[p m]

just changed my plugs & wires and resulted in an average 2 mpg increase

I am willing to bet that it is BS.
If you had so many misfires due to shitty plugs and wires so you ran 15% rich on average all the time, it would have tripped CEL.

 

[Le shed]

O2 sensors. Just went through same thing.
Bill B

 

[Tugela]

Could also be seasonal gasoline quality. Last winter in the northwest lower quality gasoline flooded the market out here. I had a few friends report lower fuel economy and one even took his truck in to a shop to have it checked. The tech returned the truck with a clean bill of health and diagnosis of lousy fuel. Spring came, the quality of gas improved, mileage improved. I can't explain why gasoline quality would fluctuate seasonally, but it's a possible explanation for the loss in range you are experiencing.

 

[bmohan55]

I am willing to bet that it is BS.
If you had so many misfires due to shitty plugs and wires so you ran 15% rich on average all the time, it would have tripped CEL.

Not BS. also replace an O2 sensor but that only tripped the code about a week before the work was done. Went from 14 to 16 mpg on mixed daily driving.

 

[az_max]

Could also be seasonal gasoline quality. Last winter in the northwest lower quality gasoline flooded the market out here. I had a few friends report lower fuel economy and one even took his truck in to a shop to have it checked. The tech returned the truck with a clean bill of health and diagnosis of lousy fuel. Spring came, the quality of gas improved, mileage improved. I can't explain why gasoline quality would fluctuate seasonally, but it's a possible explanation for the loss in range you are experiencing.

Around here it would be due to EPA mandated additives (ethanol). Or the guy at the corner market adding to his profits by watering down the gas.

 

[Blue]

I average about 11-12 mpg. Maybe 13 mpg on an easy highway cruise with the wind at my back. Dropped maybe 1 mpg when I took it from stock to lift and winch bumper. Never noticed a difference even after replacing a bad O2 sensor, new plugs & wires, etc.

 

[brianhoberg]

12.6 - 13 here.

 

[p m]

Not BS. also replace an O2 sensor but that only tripped the code about a week before the work was done. Went from 14 to 16 mpg on mixed daily driving.

NOW we're talking.
This is a different statement than that -

just changed my plugs & wires and resulted in an average 2 mpg increase

 

[rjl2001]

Ok guys thanks for all the ideas. I checked air filter, tire pressure, and added some fuel conditioner at recent oil change. I'm pretty sure new plugs and wires went in when I had the head gasket done about two years ago. The 02 sensor is kind of what I'm thinking it might be. Over the last year or two I've had the CEL come on and off with a code that was related to O2 sensor. So just because the CEL is not on, doesn't mean one of the sensors could be bad and causing the decreased mpg? I'll have to read the codes again and see if its still on there to figure out which 02 to replace.

 

[p m]

Get yourself an OBD-2 code reader (slightly better than the rock-bottom-cheapest version), and see what signal do you get from O2 sensors. That would be a good start.

 

[Some Dude]

Could also be seasonal gasoline quality.

Agreed. I've been between 12.8-13.9 around town this winter and have only squeaked out 16 on one highway trip. I'm usually around 14 in town and I saw 17.5 and even 18.8 (that was on a 30 inch A/T and no SD rack) on the return trip from a trip through Montana last summer. I have a graph of all this (yeah I'm a nerd), and it's consistently lower in the winter.

 

[baddmojoii]

Ever since they started increasing the amount of ethanol in the fuel, mileage has suffered. Doesn't have the same "kick" as straight gasoline, so it takes more of it to do the same job. Part of the reason that the fuel blends change during winter it to make fuel atomize better to improve vehicle starting during cold weather.

 

[p m]

it's consistently lower in the winter.

Could it be because it is generally colder in winter in Idaho, and it takes a little more to keep that oil warm and flowing?