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Old 02-07-2013, 11:26 AM   #1
shiplemw
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Default Terrible MPG from 23mpg to 10mpg. Suggestions?

I searched and found plenty of threads regarding terrible fuel economy. This is a bit different. I've been seeing 20-23mpg for the past several years with this vehicle ('05 STI, stage 2). I consider this normal with my daily drive of 70+ miles, mostly highway. With no noticeable change to my driving habits or my drive I'm now in the 10mpg area on this tank. I don't warm my car up for longer than 30-45 seconds in the morning before my commute. Spark plugs were changed ~10k miles ago with no signs of fouling, car idles well, no white smoke, tires / wheels are in good condition, alignment was done ~10k miles ago, oil consumption ~1/2Q every 1k miles and there are no check engine lights. The only recent things are that a.) I unhooked the a/c belt because the idler was squealing and I had not had time to get parts and b.) yesterday I diagnosed the aftermarket springs in my front struts are moving so I need to take them off and re-install. Any ideas or other things to look at?
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Old 02-07-2013, 11:51 AM   #2
Dave D.
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Does it correlate exactly with the AC belt removal?

If so, it's the only variable I can see. Put it back on and see what happens.
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Old 02-07-2013, 11:53 AM   #3
WMCCook29
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I would start by checking the front O2 sensor.
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Old 02-07-2013, 12:01 PM   #4
MaddMax
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Do you have a way to watch your AFR and O2 sensor values? Something as simple as the Torque app would do this.

I don't know how long you've owned the car, but a 15-20% drop in fuel economy will happen if you live an area that has a real winter. Secondly, if you do a lot of snow driving, that too will suck out MPGs.

Regardless, you MPGs are way low. If it was in the 15-17mpg range, I'd say that's the way it goes especially running Stage II. Seeing that you've got a tune, I'd check to make sure there's not some tune setting that causing an overly rich condition during normal driving. A lot of these tunes richen the AFR quite a bit above 3000rpms compared to stock and if you drive in these rpm ranges, then you're mpgs are going to suffer.
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Old 02-07-2013, 12:06 PM   #5
KOJAK-02-WRX
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Not sure it would cause that much issue but try cleaning MAF sensor.
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Old 02-07-2013, 12:24 PM   #6
shiplemw
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Quote:
Originally Posted by Dave D. View Post
Does it correlate exactly with the AC belt removal?
It didn't exactly correlate to the removal of the belt. I thought about putting it back on but with the reduction in loading it should get better if only by a tiny amount, not worse. Something to try though.

Quote:
Originally Posted by WMCCook29 View Post
I would start by checking the front O2 sensor.
I've never done this. I'm assuming there is a test where a certain voltage should be seen at idle, etc? Any more details?

Quote:
Originally Posted by MaddMax View Post
Do you have a way to watch your AFR and O2 sensor values? Something as simple as the Torque app would do this.

I don't know how long you've owned the car, but a 15-20% drop in fuel economy will happen if you live an area that has a real winter. Secondly, if you do a lot of snow driving, that too will suck out MPGs.
I've had the car for 5 years, ~100k miles. No major dips in fuel economy like this. I have romraider and can log some data and potentially compare it to previous results.

Quote:
Originally Posted by KOJAK-02-WRX View Post
Not sure it would cause that much issue but try cleaning MAF sensor.
That's a simple one I'll try that tonight.
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Old 02-07-2013, 12:40 PM   #7
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Old 02-07-2013, 12:59 PM   #8
shiplemw
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Originally Posted by WRXSD View Post
Shouldn't a CEL come on for a failing O2 sensor?
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Old 02-07-2013, 04:44 PM   #9
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Maybe for a sensor that has completely failed, but not one that has started to sense incorrectly. I've seen plenty of these threads with the same problem. I'm not going to say I'm 100% sure, but it is a cheap (and overdue) maintenance item.
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Old 02-07-2013, 04:46 PM   #10
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Is the car slower? If it is slower and with poor fuel economy, I'd be checking compression.
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Old 02-07-2013, 05:20 PM   #11
Karlot
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lol you got 23mpg???? damn dude i only got 18 on a stage 2 on my 06sti, and that's if i am out of the pedal.
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Old 02-07-2013, 05:38 PM   #12
CarverHouseRally
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Quote:
Originally Posted by Karlot View Post
lol you got 23mpg???? damn dude i only got 18 on a stage 2 on my 06sti, and that's if i am out of the pedal.
Same, 23 seems hard to believe.
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Old 02-07-2013, 06:03 PM   #13
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Quote:
Originally Posted by CarverHouseRally View Post
Same, 23 seems hard to believe.
Not that hard. With my stage 2 not dialed in I got 26 on an all highway run. Normal use right now is around 12-15 because of that pesky 1.5 mile commute to work in the winter.
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Old 02-07-2013, 06:07 PM   #14
Fierysun
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Quote:
Originally Posted by WMCCook29 View Post
I would start by checking the front O2 sensor.
This...

Usually, if there is a sudden drop in mpg, it has to do with a faulty o2. Also check for vacuum leaks.
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Old 02-07-2013, 06:15 PM   #15
shiplemw
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Quote:
Originally Posted by Karlot View Post
lol you got 23mpg???? damn dude i only got 18 on a stage 2 on my 06sti, and that's if i am out of the pedal.
Quote:
Originally Posted by CarverHouseRally View Post
Same, 23 seems hard to believe.
I usually get between 270-300 miles / fillup (~12-13 gal). Like I said highway miles, it's flat and I cruise 70.

I'll do a test on the O2 sensor tonight if I can. Might have to wait until the weekend.
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Old 02-07-2013, 09:36 PM   #16
DerekB707
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I've gotten 24mpg on the highway using my CC at 70mph on stage 2 many times, but touch the gas pedal and it drops pretty fast. Not sure if you have an Acess Port, but checking all the things people are recommending here is pretty easy using one.
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Old 02-08-2013, 02:10 AM   #17
esteve
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Wow, so almost all highway driving and you're only getting 10 mpg now? With a 50% drop in fuel economy, I'm surprised there aren't any signs of trouble. Has the oil consumption been at 1/2 quart per 1K miles for a long time or just recently? I would also say check for any fuel leaks but you'd notice the odor.
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Old 02-08-2013, 08:39 AM   #18
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Quote:
Originally Posted by WRXSD View Post
That o2 is only for 2.5l sti's correct, so would that fit my hybrid which has a 2.5L block? Confused.. Or should i go with a sensor made specifically for an 04 2.0L wrx?
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Old 02-08-2013, 10:47 AM   #19
danger1138
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Im also thinking of switching to a new o2 since my car has 153k on odometer but 15k on built motor. I believe previous owner never changed it either.. My car eats gas like a mother*****r.
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