PeterD Posted December 22, 2008 Share Posted December 22, 2008 I have been keeping track of my fuel mileage for as long as I can remember with all of my cars. Lately I have been seeing a huge drop in fuel economy. My legacy will average around 25mpg commuting to work and running around town. On long highway drives I have seen it peak at 28-30mpg. My last three fill ups I have seen my average mpg drop from 25mpg to about 22 and one fill up 21mpg. Now the weather out here in western Washington has been quite good for Subaru owners and I have been playing alot in the the snow with the car, but after owning two legacys for a few years I have never seen my mileage drop below 24mpg even in the bitter cold of an eastern washington winter. my first thought was o2 sensor is going out, but maybe its ethenol blend fuel:confused: Quote Link to comment Share on other sites More sharing options...
etc Posted December 22, 2008 Share Posted December 22, 2008 Did you check the codes? Keep in mind that the O2 sensor may get lazy and not necessarily throw a code. I hear it is a semi-regular item to replace, at 80K as one estimate. Quote Link to comment Share on other sites More sharing options...
mountaingoatgruff Posted December 22, 2008 Share Posted December 22, 2008 my first thought was o2 sensor is going out, but maybe its ethenol blend fuel:confused: if you suspect the fuel, try using another brand for a bit and see if it changes anything. i have a close friend that works as an environmental auditor for gas stations and he's told me to avoid arco/bp fuel and that shell is just as good as chevron. i still buy the cheapo arco stuff... then there's all that logical stuff to check: engine maintenance, fuel filter, tires, etc. Quote Link to comment Share on other sites More sharing options...
nipper Posted December 22, 2008 Share Posted December 22, 2008 Blu was getting 19, 28. The front O2 sensor went bad now im getting 16, 23. Bet your front O2 sensor is long in the tooth. Ethanol fuel really doesnt do a big hit, and winter blend doesnt either, its the cold weather that comes along with the winter blend that does it. Also a lazy T-stat will give you lower gas mileage. nipper Quote Link to comment Share on other sites More sharing options...
idosubaru Posted December 22, 2008 Share Posted December 22, 2008 there's a number of threads about this, probably most due to the change in fuel blends. i know nothing about it, but lots of "gas mileage went down" threads all over the automotive forums. playing in the snow, weather, running defrost constantly (turns the A/C on in most vehicles to dry out the air), and letting the car warm up are all gas eaters. check that tire pressure. cheap tires. maintenance items up to date. Quote Link to comment Share on other sites More sharing options...
screwbaru2 Posted December 22, 2008 Share Posted December 22, 2008 I'm no scienetist but when the weather gets cold the gas is commpressed so there are more gasoline molucules in a given amount of gasoline that when the weather is warm. The result is you use more gasoilne. My MPG always goes down in the winter a little. Thats why you should always buy gas in the early morning when it's the coldest as you will get more gas for you buck. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted December 22, 2008 Share Posted December 22, 2008 I'm no scienetist but when the weather gets cold the gas is commpressed so there are more gasoline molucules in a given amount of gasoline that when the weather is warm. that's not quite true. for liquid is not compressible like air. actually the opposite is true: an engine should run more efficiently in the winter. air, being compressible, does get more dense as it gets colder. so it's easier to get more air in the engine. put an empty plastic bottle from a hot room (room temp) in the freezer with the lid on. the bottle gets crushed significantly. that's due to the air decreasing density...well actually it's a little more complicated, but close enough. do the same with a bottle of liquid..water, gas, whatever, and it won't do anything. Quote Link to comment Share on other sites More sharing options...
nipper Posted December 22, 2008 Share Posted December 22, 2008 Gas is stored in the ground at 50 degrees. Thats how warm the earth is below the frost line. Thats an old wives tail, unless the fuel is stored above ground. ALso in the morning cold air is also denser then warm air. But eht engine oil is thicked too. Usually we kill our gas mileage by letting the car warm up (as we should in bitter cold). Winter kills gas mileage, its that simple. nipper Quote Link to comment Share on other sites More sharing options...
lostinthe202 Posted December 22, 2008 Share Posted December 22, 2008 do the same with a bottle of liquid..water...and it won't do anything. except break when it freezes:) Quote Link to comment Share on other sites More sharing options...
screwbaru2 Posted December 22, 2008 Share Posted December 22, 2008 Isn't that what makes water freeze the molecules get closer together? Quote Link to comment Share on other sites More sharing options...
bheinen74 Posted December 22, 2008 Share Posted December 22, 2008 its due to the "winter" blend gas, and the fact they are mixing more ethanol in all the fuels.... ethanol is bad stuff i say. And now, almost all gas is mixed with the crap...due to regulation. Quote Link to comment Share on other sites More sharing options...
subaruplatt Posted December 22, 2008 Share Posted December 22, 2008 I remember the 70's when they didn't put any gas line anti freeze into the gas for winter. Frozen fuel lines and buying little bottles of methyl hydrate to add to every tank. Then government discovered that a 10 % blend greatly improved inner city carbon monoxide levels. Now I think that you have to pay for premium gas if you want the same energy density in the winter. Also there is that old air density problem. I always thought an intake air warming system would be a great idea, but I guess most folks are more satisfied with the extra hp they're getting. Quote Link to comment Share on other sites More sharing options...
nipper Posted December 22, 2008 Share Posted December 22, 2008 Isn't that what makes water freeze the molecules get closer together? BUt it also expands. Water is wierd. nipper Quote Link to comment Share on other sites More sharing options...
bulwnkl Posted December 22, 2008 Share Posted December 22, 2008 my first thought was o2 sensor is going out, but maybe its ethenol blend fuel:confused: Washington has required 10% ethanol in all gasoline for far longer than your last 3 tanks (unless you haven't driven in a year or so), so it's not that. My Baja is a turbo, and its mpg declines with temperature each winter, then increases with temperature each spring. I even have the paper log of every tank the last 2 winters to prove it. As far as I have been able to tell, turbo Subies are somewhat more sensitive to the cold than the N/A ones. Or, maybe that's just me... Anyway, if you have recorded your mpg in a book or spreadsheet that you can look up, make sure it hasn't done this every winter when it gets cold. There is genuine reason why this happens (cold fluids as was previously mentioned), and why it happens more to an AWD than a 2WD vehicle. Still, the O2 sensor is a possibility. I personally have not had one become 'lazy' on me enough that replacing it before it threw a code helped mpg noticeably. I have seen that happen for others, though. Check your tire pressures and general maintenance items (including air filter), check your mpg log to make sure you haven't just forgotten how much winter impacts your mpg, and then consider your front O2 sensr if you want to. Besides that, wait 'til spring? Good luck. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted December 22, 2008 Share Posted December 22, 2008 Isn't that what makes water freeze the molecules get closer together? well that starts getting into materials, properties and crystalline structures, that's a complicated scenario actually. but the basic principles for this discussion are that fluids are not compressible. this is all basic fluids stuff, i'm sure it's all over the place and easy to find online. Quote Link to comment Share on other sites More sharing options...
PeterD Posted December 22, 2008 Author Share Posted December 22, 2008 the reason I suspect a problem is due to the fact this is my 3rd winter in the legacy and the first time I have seen the mileage drop this low. Though I have never replaced the O2 sensor. Tires are good, t-stat is new, there is no A/C so using the defrost isn't an issue. thanks for the ideas, i have searched and it seems an 02 sensor could me my problem. Maybe i'll leave it alone till the weather warms up a bit. Quote Link to comment Share on other sites More sharing options...
subaruplatt Posted December 23, 2008 Share Posted December 23, 2008 If your using the defrost aren't you using the AC as well?:-\ So defrost in these cars does = AC and may be another of the reasons. Thus their are 3 main reasons as far as I can see. Fuel density being the greatest. Quote Link to comment Share on other sites More sharing options...
bulwnkl Posted December 23, 2008 Share Posted December 23, 2008 (edited) If your using the defrost aren't you using the AC as well?:-\ So defrost in these cars does = AC and may be another of the reasons. Thus their are 3 main reasons as far as I can see. Fuel density being the greatest. He said the car doesn't have A/C, so no, he's not using the A/C when he's using defrost. Since the computer is monitoring the oxygen levels in the exhaust stream (which tells you about fuel usage essentially per molecule, not per volume) and adjusting fuel injector duty cycles to get the ratio it wants, it would compensate for any fuel density variation automatically. Edited December 23, 2008 by bulwnkl Quote Link to comment Share on other sites More sharing options...
subaruplatt Posted December 23, 2008 Share Posted December 23, 2008 Good point ! No AC but cars run faster with cooler temps but use more fuel too. Quote Link to comment Share on other sites More sharing options...
PeterD Posted December 29, 2008 Author Share Posted December 29, 2008 On friday I installed a new O2 sensor, took it on a 270 mile trip this weekend and managed 26.8mpg. Considering the snow conditions and slow speed for the first half of the trip I believe the 02 sensor is what was wrong. i can usually get arond 27-29mpg if im crusing around 70-80mph. Quote Link to comment Share on other sites More sharing options...
nipper Posted December 29, 2008 Share Posted December 29, 2008 Blus o2 sensor finally reste, and my gas mileage is back up to where it should be, so yes they will do that nipper Quote Link to comment Share on other sites More sharing options...
Pontiac6KSTEAWD Posted December 29, 2008 Share Posted December 29, 2008 actually the opposite is true: an engine should run more efficiently in the winter. air, being compressible, does get more dense as it gets colder. so it's easier to get more air in the engine. More efficiently yes, but they also burn more gas. Because the air is so dense, the computer has to offset the density by increasing the amount of fuel used. The feeling that cars have more power when cold is true, to a degree, because the BTU level inside the combustion chamber is now at a higher, more efficient level, thru both higher gas levels, and the density of the air. Turbocharged cars, like my McLaren love the cold air, but hates the traction that snow gives LOL.. Quote Link to comment Share on other sites More sharing options...
nipper Posted December 29, 2008 Share Posted December 29, 2008 Turbocharged cars, like my McLaren love the cold air, but hates the traction that snow gives LOL.. :eek: OUT you dont belong with us pesants! heheheheheeh Quote Link to comment Share on other sites More sharing options...
Pontiac6KSTEAWD Posted December 29, 2008 Share Posted December 29, 2008 :eek: OUT you dont belong with us pesants! heheheheheeh Maybe I should change that a bit.. My McLaren powered Pontiac Grand Prix.. 3.1 TurboCharged intercooled engine, pushing roughly 410 HP/393 TQ (non-stock). Quote Link to comment Share on other sites More sharing options...
PeterD Posted December 29, 2008 Author Share Posted December 29, 2008 Maybe I should change that a bit.. My McLaren powered Pontiac Grand Prix.. 3.1 TurboCharged intercooled engine, pushing roughly 410 HP/393 TQ (non-stock). I hearby vote you out of this thread:mad: please leave your off topic comments out of my thread:) Quote Link to comment Share on other sites More sharing options...
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