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Discussion Starter #1 (Edited)
Hey everyone. I recently just joined and have been coming to some odd problem with my WRX. Before anyone jumps on me about it I know the WRX is not meant to get great fuel economy. Driving in Socal when driving like a maniac I know that the fuel will drop off but I am having a certain issue with the fuel economy. When I bought this car about 3 months ago I did an overhaul and had a mechanic check everything and he said it all checked out. The car was and is still completely bone stock. I am having this issue with severely poor fuel efficiency. Had the spark plugs swapped and recently did the oil change with Castrol Syntec. Cleaned out the MAF and replaced the air filter to ensure there were no issues in regards to the MAF sensor. Her clutch, thermostat, timing belt major maintenance was done at around 83K miles. There were no apparent or modifications that were done on this car, what bothers me about this is that my mechanic pulled the codes and after having the spark plugs changed, there were no codes to be pulled. So my quandry is not the fact I am getting poor fuel economy, it's that I am getting such crappy fuel mileage.

After being around and researching what kind of mileage I am getting, I decided to log my mileage and have come to about an average of about 17-20 MPG. In regards to how I drive, I've been very conservative about driving my car and unfortunately, even with my light foot I'm still getting well below the mileage I would have come to expect. Averaging around 65-70MPH on the highway, shifting at 3K-3.5K RPMS and not dragging anything heavy in my car except for a floor jack and a few tools. Doing about 80/20 highway/city driving I would have figured the numbers would be better. Is this to be expected on the WRX? I know that if I wanted great fuel economy I would have bought a Prius, but I figured there would be considerably better fuel efficency than what I am getting. Is there a possiblity my O2 sensor has gone to crap? What could be causing this and is there anything I can to do alleviate this issue, before resorting to tuning or modifying anything. My car is completely bone stock, no modifications to speak of. Any help from the Gurus of Club WRX would be greatly appreciated.

Note: One thing I have noticed is that when I use a gas station that is not Chevron, as in a generic Arco or no name gas station, the car seems to be burn off the fuel faster than expected. Whereas the chevron seems to burn longer. On average, I see that the Arco gets around 200 miles to a tank whereas the Chevron can get upwards of 270-280.

Edit: It's a 2004 WRX Sedan. Sorry about that lol.
 

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You made it through all of that and didn't mention what year the car is. :rotfl:

That said, I only get 20-22mpg in my bugeye, and I don't drive Vin Diesel style. I did notice my mileage drop into the teens just before I threw a bad O2 sensor code. I replaced the O2 sensor, and I'm back in the 20-22 mpg range.
 

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it says your getting around 270-280 on a tank of gas. thats about as far as i drive until it needs to get filled up again, but that is before my gas light goes on and im around 23 mpg and lowest ive gotten is 17mpg. and since your doing 80/20 highway/city driving you should definitly be in the 20's, id think around 25 with your conservative driving style. my front 02 sensor keeps "malfunctioning" my code reader says but havent noticed a drop in the gas mileage. just wait and see what other members have to say. not sure what your problem. since the car is stock, maybe the previous owner put a tune on it and its getting more fuel then needed?
 

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Assuming that your MPG is accurate as per above:

What year is the car? If it's 06 or newer, I should just mention that both the front and the rear o2 sensor have a bearing on closed loop fueling, earlier cars only use the front.

First: is the car tuned at all? Should it be tuned (as in, do you have an intake, exhaust, etc. List all mods)?

Air-fuel ratio is (for closed-loop, i.e. cruising, highway sub ~65mph, idle, when not in boost) guessed by MAF sensor input, then dialed in with the o2 sensor(s). It sounds like your MAF was just cleaned, although this could be an issue still (mine would not get clean, so I went speed density :rotfl: ). Otherwise, it could be that your o2 sensor(s) are wrong, and forcing the engine to add more fuel when it shouldn't. The only way really to tell would be to throw a known-good wideband o2 sensor on, telling you exactly where your AFR's are at all times, unless it got bad enough to where it started idling rough or smelling rich.

Otherwise, you'll get bad mileage if 1) you're running a hell of a lot more ethanol than you think (e10 is at many pumps, and can lead to worse-than-average mileage), or 2) you're getting knock. If for some reason the effective octane of your fuel is less than it's supposed to be (91, as you know), this can lead to knock, which pulls timing, which makes each combustion event less efficient, which makes the overall car less efficient. Assuming you're running premium gas, check to see that an inordinate amount of oil isn't coming in through the PCV system, and do a compression test. Check your spark plugs, it's normal for them to look like the mixture is rich (WRX's and most turbo cars swing rich under boost, crazy rich in stock tune), although check for significant amounts of other contaminants, especially one cylinder worse or different than the others.

If all that checks out, then it's just your car. If you get a tune, with or without other mods, you'll naturally see a great increase in mileage, and most likely whatever fluke is causing your bad mileage will be tuned out as well.
 

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Op has an 04' wrx. I consistently get 22+ mpg in the city and 27+ freeway. I have an 09 WRX, bone stock. I shift at 2500, if you stay out of the boost range then you get 4cyl MPG, if you're in the boost, you get thirsty V6 MPG.
 

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Discussion Starter #9
There are no mods to speak of on this car. Not sure if the ecu has been flashed or messed with. My suspicions are that it's due to the MAF either being dirty, an O2 sensor going bad or possibly the ECU being messed with.
 

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LOL, you're all a bunch of Leadfoot Larrys. I still get roughly 24ish mpg with a hybrid engine w 18G, and got 27ish on the stock engine, regardless of what map was in it. GRANTED...I do a fair amount of highway on my commute, and I cruise at around 70. Going 80 will only get me to the next slowdown a minute or two sooner...
 

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There are no mods to speak of on this car. Not sure if the ecu has been flashed or messed with. My suspicions are that it's due to the MAF either being dirty, an O2 sensor going bad or possibly the ECU being messed with.
The 04' is rated at 18/25 mpg. Seriously, you aren't that far off if you're averaging around 20. It could be something as simple as your tires need more air, are you running the a/c? If you were driving a prius and only getting 20mpg I would be worried.

Judging by your name I'd guess you don't drive like a grandma all the time, which is required to get the rated mpg, especially the pre 08' mpg guidelines which were even more misleading to begin with than todays ratings
 

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Discussion Starter #12
Updated: After burning through a few tanks of gas. The mileage is reading an average of 19MPG on average with a 80/20 commute on the highway. So something is very wrong. I'm thinking of taking her to a tuner and getting her reflashed back to stock. Also going to try and test the O2 sensor later and see what kind of results turn up.
 

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Updated: After burning through a few tanks of gas. The mileage is reading an average of 19MPG on average with a 80/20 commute on the highway. So something is very wrong. I'm thinking of taking her to a tuner and getting her reflashed back to stock. Also going to try and test the O2 sensor later and see what kind of results turn up.
Where are you shifting? How hard are you on the throttle? Do you have a lot of hills? 19 is really not that unreasonable for these cars. This last tank I've been driving extremely conservatively and I am still just getting 23.4 on the car's readout (which is not 100% accurate either, btw).
 

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Updated: After burning through a few tanks of gas. The mileage is reading an average of 19MPG on average with a 80/20 commute on the highway. So something is very wrong. I'm thinking of taking her to a tuner and getting her reflashed back to stock. Also going to try and test the O2 sensor later and see what kind of results turn up.
how will you test the 02 sensor?
 

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Discussion Starter #15
The points I'm shifting are at between 2.8K to 3K RPMS. This last week I have not hardlined the engine or drove it hard. And to answer your question Ruh Roh, what I'll have to do is have a friend of mine take the sensor out and put it on a wideband O2. A friend of mine who deals with these a lot told me that it's possible the sensor is working but not fully and it's working lazily. He suggested taking it to a shop and running it on a dyno with the smog machine. Put the evaporator on manual and see what comes out or something. I'm gonna run it by him and see what I can do.
 

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Where are you shifting? How hard are you on the throttle? Do you have a lot of hills? 19 is really not that unreasonable for these cars. This last tank I've been driving extremely conservatively and I am still just getting 23.4 on the car's readout (which is not 100% accurate either, btw).
Yeah, I hand calculated my last tank, it was about 20.x MPG while my readout said 21.x.

I've been driving about 80% city/20% highway and I'm still getting around 20MPG. I only drive about 200miles a week, so I end up filling up every week and a half.

My wife's MCS got about the same mileage around town and it had around 100 less HP. So I think we're doing OK. :)
 

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Discussion Starter #17
After having my mechanic pull the codes. He had tested the engine once again and the code was reading P0304. Seems my information was off and so the fourth cylinder is misfiring once again. Seems I might have to get a compression test done and get the head possibly be rebuilt. Unsure of what else beyond that but to make a claim with my warranty guys and see what they can do to fix it.
 

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LOL, you're all a bunch of Leadfoot Larrys. I still get roughly 24ish mpg with a hybrid engine w 18G, and got 27ish on the stock engine, regardless of what map was in it. GRANTED...I do a fair amount of highway on my commute, and I cruise at around 70. Going 80 will only get me to the next slowdown a minute or two sooner...
Werd to that. I get 24mpg city average and I tend to be pretty leadfooted at times. Been as high as 31 highway as well. Typically see 29 though.
 

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wow ok, I really need to jump in on this conversation. I recently bought my 2006 WRX TR and i already know it holds 15.9 gallons of fuel and im getting absolutely horrid milleage. this current take that im on (all city driving) i think im getting like 13.8 MPG. My fuel guague is about at 1/8th tank right now so my light is about to come on and ive only driven 210 miles. From what i read thats about 70 miles short of a happy meal. Im pretty positive the car was not messed with by the original owner, had the SPC air intake on it before and i will be reinstalling it again. what on earth would make me get such bad MPGs?! i shift at 3 to 3.5k and i dont go stomping it everywhere... im about to drive about 280 miles on the highway next week when i drive home...so that should give me a good reading as well...

someone...
anyone...
please...
lol...
 

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wow ok, I really need to jump in on this conversation. I recently bought my 2006 WRX TR and i already know it holds 15.9 gallons of fuel and im getting absolutely horrid milleage. this current take that im on (all city driving) i think im getting like 13.8 MPG. My fuel guague is about at 1/8th tank right now so my light is about to come on and ive only driven 210 miles. From what i read thats about 70 miles short of a happy meal. Im pretty positive the car was not messed with by the original owner, had the SPC air intake on it before and i will be reinstalling it again. what on earth would make me get such bad MPGs?! i shift at 3 to 3.5k and i dont go stomping it everywhere... im about to drive about 280 miles on the highway next week when i drive home...so that should give me a good reading as well...

someone...
anyone...
please...
lol...
Did the guy you bought it from have the intake on it or did you put it on? If it was on there it might be tuned for the intake, so having it off might be bad. I'm nit an expert, I'm not even a novice. Just a guess
 
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