Fuel Economy Decrease

[gtosteve65]

I have a 2006 mercury mountaineer awd v8 which up until a few weeks ago would be able to get 22 mpg highway. I have noticed that recently it has been around 17 mpg at the same speed. Could all of this be blamed on winter gas blend, or is there more to my problem?

 

[Carguy3J]

Much of it can probably be attributed to the cold weather. (Actual air temp drop) I think that has a bigger impact then any change in the gas itself. Colder air is denser, meaning it has more oxygen content. That means you need a richer fuel mixture (more gas). While it can make more power as well, fuel economy suffers.

Also, people tend to let their vehicle idle more in the cold weather, also burning more gas.

Finally, in bad weather, even just rain, the vast majority of drivers get stupid, and drive needlessly slow, resulting in more stop-and-go traffic, and less fuel efficient driving styles. Personally, it really pisses me off that MY fuel economy can vary 2-3mpg sometimes, depending on how OTHER people force me to drive! Its only made worse in bad weather. If your little import crap-box can't go more then 5mph on a snowy road (or you just don't have the driving skill to do so), then stay the hell home!

So, as long as everything else seems normal (no change in drive-ability, no CEL,etc..), then I would blame it on the cold.
Oh, also, check your tire pressure. Low tires will also lower fuel economy. Tire pressure drops something like 1-2psi for every 10deg drop in temp. Don't hold me to those numbers exactly,but it is something you need to check.

 

[Henders]

You might need to replace your plugs and your fuel filter (if you haven't yet). I also don't disagree with the previous post, about gas mileage dropping in the winter.

 

[gtosteve65]

The fuel filter was done this summer. Im gunna check the plugs when I get home next week (im in college in rochester). I have noticed people making me drive slower but even on days where I can drive how I used to during the summer, my gas mileage is so far off. Tire pressure is good 35psi in all tires

 

[Henders]

If you're talking about Rochester, NY, I know that place well. In local driving, my mileage there dropped down to about 15 MPG in the winter (mainly due to me not driving on the highway). The high gas mileage comes from the last two gears, anything else literally sucks!

 

[F SANE IL]

Just noticed yesterday that my MPG went from 18 to 13 since it got colder. Aldo the car feels a bit slugish and the damn belts squeak.

I blame the cold

 

[gtosteve65]

Well im basing the highway mileage off of my time on the major highways. I reset my mpg counter once i get to cruising speed, but it can never get above 19 anymore. Used to be 22!!

 

[Henders]

Still sounds like its your plugs. I remember my 06 EX drooping in MPG on my trips from Buffalo to Rochester to about 17MPG, didn't think much of it, I think I had around 75K on it. didn't throw any codes until around 115K. After I had the plugs replaced, my MPG went back up to 22 to 24 on that trip. You also have to remember that the environment also plays a part on the mileage. wind to your back=Good, anywhere else=bad.

 

[Flag Gibby]

I personally blame it on two things: the outside temperatures and the high ethanol content in the winter gasoline blends. My vehicle is down to 15.5 MPG on this last tank because it hasn't gotten above freezing at all this last week until the other day and the vehicle is having a hard time getting up to operating temperature (I don't idle the vehicle because it is parked in an insulated garage and is already at around 50 F so no loss from idling). The operating temperature part may also indicate that my thermostat is going bad again since it cannot maintain engine temperature very well at all. However, when the ethanol content in gasoline goes to 10% my fuel economy always goes down on the vehicle too.

 

[Carguy3J]

I personally blame it on two things: the outside temperatures and the high ethanol content in the winter gasoline blends. My vehicle is down to 15.5 MPG on this last tank because it hasn't gotten above freezing at all this last week until the other day and the vehicle is having a hard time getting up to operating temperature (I don't idle the vehicle because it is parked in an insulated garage and is already at around 50 F so no loss from idling). The operating temperature part may also indicate that my thermostat is going bad again since it cannot maintain engine temperature very well at all. However, when the ethanol content in gasoline goes to 10% my fuel economy always goes down on the vehicle too.

In NJ, and many other states as well (I think), we have the 10% ethanol all year round now. IF you build and/or tune your engine for it, it can be a good thing. Unfortunately, when its forced on vehicles not optimized for it, it definitely reduces fuel economy. This is due to the lower stoichiometric ratio of ethanol, vs gasoline. While gas is 14.7:1, I believe ethanol is 9:1, meaning it requires a richer mixture (more fuel).

A cold engine, while slightly more powerful, will get lower fuel economy. Its possible that your thermostat may be sticking open, allowing for greater coolant flow then needed.
You may also want to check your fan clutch. If that is worn and allowing the fan to turn more then needed, again that will keep the engine cold, as well as robbing engine power through parasitic drag. That will, in turn, also contribute to lower MPG in its own way. You should consider getting rid of the mechanical fan, and installing an electric fan. There are plenty of threads here on the site about how to do it. It is worth about 10-15HP and roughly 2-3 mpg average improvement.

 

[Flag Gibby]

4th gens. do not have mechanical fans they have an electric clutch on them. I have fought this issue on this vehicle once already with the thermostat sticking and not able to effectively maintain optimal operating temperatures and simply replacing it fixes the issue. As far as ethanol content goes, the more there is the worse your MPG will be. However, you will not notice it if the percent content always remains the same.
Edit: I should have called the fan an electronic viscous fan and do not need to be replaced unless there is a DTC being thrown by the unit (this info is directly from Hayden, the largest manufacturer of EV fans).

 

[Henders]

Thanks for the info, I'll keep it in mind if/when my Ex acts up. Knock on wood, 130K and running great!