I'm not sure if this is the right forum to post... so if it's not the right place, please move it. I just wanted to ping this off the community. In the past 2-3 months, my mpg appears to have dropped from 20-22 mpg to about 18 mpg. I don't think it's any of the following: tire pressure gas air filter engine oil driving style I'm starting to suspect the O2 sensor as none of the above (besides driving style) would drop my mpg efficiency by that magnitude. my O2 sensor was replaced early on as faulty when I kept on getting a CEL light after my turbo gauge was installed (by the dealer.) My car is still on warranty, but before I take it into the dealership, anyone know how they (the dealership) might figure out if my O2 sensor is outta wack without a CEL light? The most annoying thing would be to go in there and have them say everything is fine. FYI, I'm driving an MY04 WRX Wagon bone stock except with KUMHO ECSTA ASX tires. Thanks in advance.
Might want to replace the fuel filter depending on your mileage. You would be surprised how much crap is in it after 30k miles!
Thanks for the reply. Fuel filter was changed in January during my 15k mile service. Air filter was changed during that time as well. It was done by the dealer. I'm a total novice here... can someone point me to a thread where it teaches me how I can tell if the air or fuel filters were changed correctly? I'm just coming up to 20k mile right now. From the below link, I suspect the dirtiest filter could yield to a 10% drop in mpg, but I'm guessing that's probably not my case... http://www.fueleconomy.gov/feg/maintain.shtml
here are my last mpg's. Date/Company/odometer/gallons/MPG/price per gal/driving style 2/24/07 QT/214.8 mi/11.470 gal/18.7 mpg/$2.399/granny - all city 3/23/07 Amoco/266.9 mi/14.730 gal/18.1 mpg/$2.729/granny - all city 4/11/07 Chevron/192.2 mi/11.577 gal/16.6 mpg/$3.099/moderate – all city now i'm getting annoyed. compare this to what I used to get which was always north of 20 mpg.
Man, I give it the boot almost everyday and still get 17.x mpg. Clogged injectors could be messing you up too.
I agree with you on the O2 sensor, my mileage is horrible since i installed my catless system........ I have to get a tune, CEL fix, replacement O2 sensor. Will this possibly fix the problem???
SOP O2 sensor, but that was done WAY back when I first bought the car... Another added piece of info is that I just past my emissions test.
notice that you're also driving an STI. I'm driving a WRX... granny style for the last 3 months and 3 gas tanks...
gasoline mixture? Here's something interesting. I spoke to my dealer at Royal Subaru of Decatur. We were chit-chatting about whether it was worthwhile to bring the car in for service. He asked me how the engine was running? It runs fine. He asked me if the CEL was on. I said no. He then said honestly, it will be difficult for them to determine the problem. I mentioned cleaning the air filter and possibly the spark plugs or even checking the O2 sensor. He mentioned that those are all possible... He didn't think it would be the O2 sensor because even if codes were thrown that don't turn on the CEL, to his knowledge, none of those "stored" codes affect MPG. He then loosened up and said he'd been getting a ton of calls these past few months regarding reduced MPG up to 20%. He thinks it's the new gasoline/ fuel mixture requirements that came into effect recently due to Fed Regulations. The Atlanta Metro Area falls under the stricter requirements. He indicated that the fuel formulation was changed in Nov 06 and in Mar 07 again. He said if you fuel up way outside of the 17-or-so affected counties, he thinks you'll still get the old MPG. I'm gonna call SoG and Classic tomorrow to see if they've seen the same thing or if I was led down a wild goose chase. In the meantime, can I get members to watch their MPG and see if they get better MPG with gas outside the metro ATL area vs inside? Thanks! UPDATE 4/12/07 I just called SoG this morning and spoke to a friendly guy on the other end... He said the following: Yes, the fuel mixture has changed with more sulfate in ATL, but he doesn't think it would affect my MPG to that degree. His susgestions fall in line with what ppl here already suggested. Replace each of these parts one by one, so I can figure out what it is: 1) tire pressure and oil filter/change already completed 2) air filter 3) fuel filter 4) clean spark plugs, but that is unlikely with my current mileage 20,000 4) o2 sensor only if CEL light is thrown And finally, he hasn't been getting any calls regarding lower MPG, so there you have it...
I would say its more likely a change in gas / atmospheric conditions more than anything mechanical. Drive it like a beast for the next tank and see if your gas mileage changes much, if it doesnt then you at least know you can have more fun on a daily basis.
I noticed when filling up today @ a different station today it had a warning label about "10% ethanol". My usual station doesn't have that warning, so I'm curious to see if it will next time.
Some of my worst MPG came from when I set my AP to "economy" and took it easy. Plus, when it's cold overnight my car seems to burn more fuel until the engine warms up. So if it's cold out, you're starting and driving a few blocks then turning it off, sitting in traffic, and doing mainly low speed city driving I wouldn't be worried about it. Another factor to consider is the amount of gas you're actually burning and miles being recorded. If you want to make it exact, ONLY put in 10.0000 gallons every time. Air in the tank, parking on a hill while filling, pump shut off... it's all changing your math. You should need to refill every 180-200 miles regardless of what your gauge says. Good luck! What are you doing driving like a granny?!
^^^ Thats what I'm asking. If I drive like a granny I get 18-19mpg, if I have fun with it I get 16-17mpg and if I'm really romping on it I get 12mpg.
I calculate my mileage every time I gas up. It's never changed more than 2 MPG. I've been in Gwinnett for--ever. (or so it seems) Uh, it does drop when I go to Road Atlanta. Even there, I've gotten better than 4 MPG.
The EPA makes Atlanta switch to a different fuel mixture every summer to combate the effects of the heat and smog. Personally I never get more than 15 to 16 mpg....4 at the track. The STi sucks......literally.
i think i've pinpointed my problem. air filter and not knowing what my car really got in ALL city driving. http://www.wrxatlanta.com/forums/showthread.php?t=12800 thanks for the help fellas.
this is just a follow up to the thread. MPG is dropping again. My monkey throwing darts at the board says that mmtasty's post may be spot on. it's been pretty cold down here lately and most of my driving has been in-city, short-trip, and short-startup-time drives... MPG's gonna be ugly on this tank, closer to 18 MPG on a stock WRX wagon.
Ding Ding Ding we have a winner. Nothing is wrong with your car, the problem is with our gas stations now. Most are switching to a 10% ethanol mix. While its not a huge difference, it is enough for you to notice a drop in your MPG.
hey Scott, took your fun class at Talladega in May 06. i was the asian guy with the stock black wrx wagon with the conservative driving. anyway, care to elaborate on your rear O2 sensor theory? if it wasn't operating within factory parameters, shouldn't it throw a CEL? the reason i'm asking is that i've always thought my MPG has been low for my driving habits compared to others. yes yes i know it's tough to do apples-to-apples, but i've been slowly moving to some theory that my O2 sensors are out of wack, but everytime i throw that theory out there, someone throws the CEL question back. since this is more of an annoying problem (and I could be wrong), i'm not planning on resetting the ECM unless i get more opinions on the O2 sensor theories. thanks!
doug, would it explain drops from 19-20 MPG city driving down to 16-17 MPG in a stock WRX? for the sake of the argument, assume tire pressure, driving style/habits, all the easy stuff have been the same and only the temperature and the fuel mixture has changed. any feedback is welcome. i just have this gnawing feeling that >10% drop in fuel efficiency for those reasons seems like alot. of course i have no one to compare this to.
without a working rear o2, could the car pass its emissions test? when this was happening last year, i had just passed an emissions test to register the car. thanks.
If the rear o2 is not working right the only result is that EVERY time it goes from open-loop (on it, or full lift) back into closed-loop (cruising) it targets 13.5 instead of 14.7 for THIRTY OR FORTY seconds... that is the only symptom at first. Siegel
I had the same problem and Siegel told me rear O2. I was watching my Wideband and it was sitting at 13.5-13.8 AFR for anywhere from 20 seconds to 1 1/2 minutes then it would jump to 14.7-15.0 Bought a new rear O2 and bam! Everytime no matter what I do it goes right to 14.7-14.8 immediately when I cruise. MPG went from 18-20 mpg to 25+.
thanks for the feedback / rear O2 test. i'll definitely keep that in mind. my commute is really short. 9 miles round trip so i've just been living with it, but with gas coming up to $4/gal, it's good to know i've got some options to check out. rear O2 will be at the top of my list.
No the rear o2 controls the fuel trims as well as checks the cat efficiency. The Cobb and most all EM's turns off the cat part if selected but the fuel trim part is still working. It's also not a cheap part....$110.
say, scott (and coolrex), i just spoke to jill (service dept) at SoG about seeing if the service dept there has any tools to measure the AFRs you apoke about. she mentioned that actually you and the performance guys were the only ones that could do it there. do you have time if i brought my car in to check it out? since the o2 sensor is not exactly a cheap part, i'd like to make sure there is a problem before having a mechanic switch it out. if you don't, i may just pull the trigger and switch it out anyways. with my next tank of gas at +$50, it's only 4 tanks to a new rear O2 sensor. --- side note --- since my car is still on warranty until august, i asked jill if this part could be covered under warranty if we/i could document the fault. her answer was two fold: (1) since a CEL wasn't thrown, subaru will likely indicate that the part is operating within parameters (2) however, SoG would certainly approach subaru with any evidence that the part was not performing to designed specs.
How modified is your car? Any mods can void that as Subaru can say that part stresses the engine and cause the O2 to fail. I doubt that they will cover that on a non-stock car. I'd help if I could but I have my own wideband that is permanently in the car However Scott or Doug at Topspeed can throw on their Innovate WB and help you.
totally stock except for a K&N air filter. changed the stock filter out when i was going down the list of potential MPG reducers. thanks.
hmmm... this may also be helpful. http://www.fueleconomy.gov/feg/calculatorCompareSideBySide.jsp?column=1&id=19891 my fuel economy after measuring for about 6000 miles averages 21.8 MPG with mostly city driving. that seems to be in line with the new EPA estimates. i'm thinking i'm jumping at shadows again.