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Old 01-29-2013, 11:20 PM   #1
Inversion
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Default Gas Mileage Problem :\

When I first got my 01 rs I was getting around 25 mpg in the city
In the past month this has rapidly deteriorated to around 15-17 mpg-I drive pretty conservatively as well shifting before 3k and I only get on it every now and then
I cant seem to figure out whats wrong :\
No cel to go off of
Fuel filter, spark plugs-gapped to about 44, wires, coilpack, front o2 sensor have all been changed out when I first bought the car
Fuel pressure also seems good ~36 psi at idle
Compression is even on all 4 cylinders
The car seems to run good, the only thing ive noticed is a slight drop in performance all around but more so in the low end than the high end.
Any ideas? I do need to do a valve adjustment, can that cause such a drop in mpg/power?
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Old 01-30-2013, 12:37 AM   #2
mod maniac
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The valve adjustment would help but it is probably winter gas.
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Old 01-30-2013, 02:28 AM   #3
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Quote:
Originally Posted by mod maniac View Post
The valve adjustment would help but it is probably winter gas.
+1

I agree.
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Old 01-31-2013, 03:41 AM   #4
s1eepR
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Winter gas.

I got 25-27 combined in the summer. (I drive like a granny 90% of the time--I came from a Pathfinder and spending 80$ a tank forced me to learn)

Winter I struggle to get 20 combined. I get maybe 24 hwy.
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Old 01-31-2013, 03:43 AM   #5
cal_look_zero
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+ juan.

Winter gas sucks. Iso-heet helps a bunch for me, like 3mpg better in the winter.
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Old 01-31-2013, 04:03 AM   #6
s1eepR
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Where can this Iso-heet be purchased? I presume it goes in with every full tank?
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Old 01-31-2013, 03:16 PM   #7
cal_look_zero
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They sell it everywhere; even grocery stores.

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Old 02-01-2013, 07:05 AM   #8
Charlie-III
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Quote:
Originally Posted by Inversion View Post
When I first got my 01 RS I was getting around 25 mpg in the city
In the past month this has rapidly deteriorated to around 15-17 mpg-I drive pretty conservatively as well shifting before 3k and I only get on it every now and then
I cant seem to figure out whats wrong :\
No CEL to go off of
Fuel filter, spark plugs-gapped to about 44, wires, coilpack, front o2 sensor have all been changed out when I first bought the car
Fuel pressure also seems good ~36 psi at idle
Compression is even on all 4 cylinders
The car seems to run good, the only thing Ive noticed is a slight drop in performance all around but more so in the low end than the high end.
Any ideas? I do need to do a valve adjustment, can that cause such a drop in mpg/power?
How long have you owned the car? It sorta sounds like not very long?

How many miles have you put on it?

Yes, if you haven't owned it long, you're likely seeing the negative effect of "winter gas."

Also check for a dragging brake caliper.

All the other items are possible, but I'm suggesting some "free checks".
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Old 02-01-2013, 08:13 AM   #9
Bababou
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I've seen ISO heet but living in central AZ I have never had issues with winter gas until now. I went from old Chevys to Hondas to my first Subaru and this is the first one to have a drop off in mileage. Are there any issues in the long run using ISO, I’m skeptical on additives after seeing what some do to engines.
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Old 02-03-2013, 11:54 AM   #10
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I don't mean to turn this into a fuel additive thread, but just figured I'd add my two cents. I'm a big fan of Chevron Techron Concentrate Fuel System Cleaner. When I first used it in my '96 OBS for a somewhat uneven idle, it cleared it right up. I've never used ISO Heet, so I can't comment there; otherwise, with that kind of sudden drop in mileage, I second Charlie-III's suggestions about checking for sticking calipers and other such things.

And this may be completely irrelevant, but the oil separator plate on the back of my engine was leaking oil onto my clutch when I first bought the car and all of the clutch slipping caused awful fuel economy. Not sure if they were still using plastic oil separator plates in 2001, but the replacement was steel.
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Old 02-06-2013, 03:50 PM   #11
Chknwgn
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Didn't realize winter killed out gas efficiency so much. I've been choosing to put 93 instead of 87 into my 2.2L OBX because its so much peppier and my MPG is considerably better.
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Old 02-06-2013, 09:59 PM   #12
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do u have a rotten egg smell coming from the exhaust?
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Old 02-06-2013, 10:19 PM   #13
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i haven't seen much benefit to using higher octane in the NA imprezas. in fact from AFR gauges car runs richer with lower 87 octane and has reasonable torque at midrange. i notice on higher octane it runs too lean ~13.4 at 4500rpm vs. ~12.8. There does seem to be higher ignition timing with higher octane but only in hot weather. not sure why but i think the intake temprature sensor has something to do with it. i know hondas have map tables for intake temp vs ignition timing. maybe subaru does as well.

as for MPGs i always get ~23mpg city and ~33mpg highway in summer and about 5mpg less during winter for the same route. right now doing lots of city driving i'm averaging 22mpg. not unusual to get 17mpg if i idle too much with frigid temperatures here in Canada. plus winter gas contains butane which evaporates in your tank to help with cold starts. so expect 10% less fuel efficiency just because of gas alone.

driving conservatively is not necessarily a good thing -> During acceleration you want to burn the least amount of fuel per amount of power generated. according to the Brake Specific Fuel Consumption chart for an EJ22 NA subaru engine, shifting at high loads and mid RPMs is best for fuel consumption during acceleration:


BSFC is basically how much fuel you are burning per amount of power generated. The lower the number the better. The top curve is the torque curve for the engine which here indicates maximum load per RPM. The curves below are boundaries for BSFC ranges. 190g/kw/h is best this engine can do around 1500-4000rpm at 80-95% engine load. But Manufacturers tend to overestimate their engines in this respect. Still our engines are tuned to run stoich below 4000rpm no matter what the load (at least +08 impreza 2.5i models do). Even still mechanical and thermal efficiencies usually fall above 2500rpm due to higher friction levels. AVLS engages at high loads independent of RPM (it's variable lift). Therefore i recommend you shift at ~80% engine load below 4000rpm. I usually shift at 2500rpm. Rarely i accelerate at 3000rpm, like if i need to get back up to speed climbing a steep hill. If you don't have a scangauge to monitor engine load you can use a simple vacuum gauge. The newer imprezas have a built in vacuum gauge which is labeled as a 'MPG' gauge. If i am right, then you should accelerate at a rate where the gauge is at 80% of maximum reach. Also try a lot of coasting and some pulse and gliding. I also turn off my cruise control because it tends to disable fuel cutoff below 1500rpm while coasting. hope this helps.

Last edited by Pickler; 02-06-2013 at 10:38 PM.
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Old 02-07-2013, 08:35 PM   #14
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I basically try to keep it under 2500 when trying to eek out MPGs. If I need to get there fast...I just watch the Avg MPG(as inaccurate as it is) go down like a bomb timer. .
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Old 02-08-2013, 08:35 AM   #15
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Quote:
Originally Posted by Pickler View Post
according to the Brake Specific Fuel Consumption chart for an EJ22 NA subaru engine, shifting at high loads and mid RPMs is best for fuel consumption during acceleration
I recorded the fuel economy of my '96 OBS (MT) for almost every single tank over the three year period I owned the car. I always found 2,500 RPM to be the magic spot on that engine as well. The best I ever got on the highway was about 29.8 MPG, on the day that I finished installing my cruise control parts. As I recall, it was Spring. Now I'm really curious to go back and compare seasonal fuel economy! Over that time, my weighted average, both city and highway, was a little over 25 MPG.
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Old 02-09-2013, 12:09 PM   #16
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Quote:
Originally Posted by jarz21 View Post
I recorded the fuel economy of my '96 OBS (MT) for almost every single tank over the three year period I owned the car. I always found 2,500 RPM to be the magic spot on that engine as well. The best I ever got on the highway was about 29.8 MPG, on the day that I finished installing my cruise control parts. As I recall, it was Spring. Now I'm really curious to go back and compare seasonal fuel economy! Over that time, my weighted average, both city and highway, was a little over 25 MPG.
yep 2500rpm sounds about right. also i have noticed that when i started applying the high load low rpm method, my trip computer would indicate higher fuel consumption. however at the pump, it was not so and the computer was way off (up to 10%). i think our trip computers estimate consumption just based on airflow and not how much fuel is actually injected. that's why i don't rely on the trip computer. I measure my mileage old school with pen and paper at the pump.

Last edited by Pickler; 02-09-2013 at 12:15 PM.
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Old 02-11-2013, 03:34 PM   #17
Charlie-III
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It may be different now, but years ago BMW did some testing. Their finding was to run ~3/4 throttle & shift just past torque peak.
The 3/4 throttle was to reduce pumping losses but not get the ECU into "power mode".
Shifting just TP was to allow the engine to be in a decent operating range.

Based on a drive from NJ to UT & back (5200+ miles round trip in a week), drafting larger vehicles was best for mileage........
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Old 02-12-2013, 08:23 AM   #18
jarz21
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Quote:
Originally Posted by Pickler View Post
I measure my mileage old school with pen and paper at the pump.
Don't even need the pen if you save your pump receipt. I would use my fingernail to scratch the odometer/ tripometer reading onto the receipt, and then enter all of the information into an Excel spreadsheet later on. Have a nice graph of weighted fuel economy based on how far I went with each tank.
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Old 02-12-2013, 03:26 PM   #19
Pickler
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Quote:
Originally Posted by jarz21 View Post
Don't even need the pen if you save your pump receipt. I would use my fingernail to scratch the odometer/ tripometer reading onto the receipt, and then enter all of the information into an Excel spreadsheet later on. Have a nice graph of weighted fuel economy based on how far I went with each tank.
lol i don't actually use paper and pen i just meant to calculate it manually is most accurate. i use fuelly.com and upload my result with my cell:

http://www.fuelly.com/driver/pickler/impreza
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Old 02-13-2013, 02:58 PM   #20
GDP024
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I just added new spark plugs,wires,air filter,pcv valve, and oil change.. and im getting worse mpg as well since the cold weather started.. gotta be that winter gasoline
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Old 02-14-2013, 06:35 PM   #21
Pickler
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Quote:
Originally Posted by GDP024 View Post
I just added new spark plugs,wires,air filter,pcv valve, and oil change.. and im getting worse mpg as well since the cold weather started.. gotta be that winter gasoline
just wondering what your mileage is?
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Old 02-14-2013, 07:18 PM   #22
HUNKO
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kinda the same problem. i have a 02 ts impreza with 80k miles and when i first got my car with 67k, i dont know exactly how much mpg i was getting but it was alot better then now. now i get 250 to the tank and i drive carefully shifting at around 2300 rpm's and i drive mostly on the parkway. changed my spark plugs, fuel filter, spark plug wires. nothing seens to be wrong with the car
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Old 02-15-2013, 03:24 PM   #23
Pickler
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250mi from a full tank doesn't sound right at all. i get about 500km doing mostly city and cold temperature starts. i think that's 350mi. i started using a block heater (came standard with my impreza premium here in canada) last week and my mileage is improving.
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Old 02-17-2013, 01:40 AM   #24
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well yours is a 09 and newer and mines a 02
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Old 02-17-2013, 03:29 PM   #25
Charlie-III
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What plugs were installed?

In general, I only recommend NGK coppers. Whatever you do, stay AWAY from Bosch Platinums. They don't seem to get along with Subaru engines.

I run my 1998 Legacy GT (2.5L) with some mods, I tend to get ~300 miles/tank (~21MPG) and more like 23MPG if I'm "easy" (still cranking on the highway....easy is a relative term).
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