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Old 05-11-2009, 05:52 AM   #1
aberg12012
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Default Bad Gas Milage - Any not so common causes?

I say "not so common" because I've gone through everything I normally would with a car that is getting lower than normal milage. New plugs, new plug wires, air filter, checked brakes (including parking brake) to be sure they arn't dragging, etc. Everything seems normal. Motor (with 80,000 miles) is not burning or using any oil, seems to have normal power (my butt dyno is very inaccurate) and starts and runs perfectly with no issues. Well, no issues (like cold piston nock) that aren't perfectly normal for a suby.

It's a 03' Baja sport (N/A) with 80,000 on it. I've owned it since 2006, and am the 2nd owner. I change oil and filter every 2K miles (dino 5w30) and have changed all other fluids (ATF, front/rear dif, etc...) shortly after I purchased the car at 50K miles.

It always used to get 24 mpg at a minimum in town/tearing around and 30-31 on the highway. This winter I started notticing it was only getting 14-18 on average. And that was mostly from the 10 mile drive to work, on a county road going 50-55 mph with no stops. I live in central WI, and it was a cold winter, which does suffer millage a little, but not that much...

So it warmed up for spring (on time this year! yay!) and I changed plugs, plug wires, and checked everything over short of doing a leak down on the motor. Still only getting 18-19mpg with mixed driving, again most of which is a 10 mile drive to work on a county road. All highway it's barely getting 23-24. So it's definitely getting 6mpg less than it used to, which is a huge drop, if I might say so myself as an amature wrench head.

The car does have a short ram air intake, with an oversized K&N cone filter which I installed shortly after buying the car. But it had been getting the same millage with it, so I doubt that's the culprit. I have checked the intake, including the TB for any debris or animal nests... it's all clean.

Is there something else that might be a common, yet not so common cause of this? FI system, O2 sensor, computer? Or does this simply sound like a motor getting tired? (Even though it seems to have normal power and not burning oil?)

Since I'm mostly a bike guy I have only a very basic understanding of how the FI system works on this car, and if this may be an issue with the O2 sensor or computer running the car too rich? I dunno. Just looking for general ideas of where to start.

Thanks
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Old 05-11-2009, 11:54 AM   #2
chazly413
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Try cleaning the MAF. I let the car idle for 2 or 3 minutes before I drive off in the morning. Different transmission fluids definitely affect your gas mileage. I recently switched back to the OEM Subaru fluid and saw a 2mpg increase. Also, try not to use cruise control. I noticed a .5-1mpg increase by not using cruise control.

I just cleaned my MAF the other day after putting my paper filter back in so I'll see how that affects this current tank of gas.
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Old 05-11-2009, 02:29 PM   #3
treksofar
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Try mid-grade gas and see what happens. My 2006 2.5i started getting crappy milage with regular recently and it seems to run happier with mid grade. If the pervious owner beat the snot out of the engine, you may have worn piston rings causing exessive blow-by, resulting in a less efficiant energy conversion.
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Old 05-13-2009, 09:08 AM   #4
aberg12012
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In any case, I see no reason why my gas millage would suddenly drop from 24 minimum, to 20 max. It just doesn't make any sense. I've always used the cruise, I've always had synthetic ATF, always had the short ram, and it's always gotten better millage with the worst of me behind the wheel, than it does now with me in my best behavior.

The only thing I can think of, is this winter I slid into a deep, snow covered ditch. I knew I wouldn't make it around the curve, so I basically aimed the car for the ditch, and kept on the gas to make it as far as I could. Ended up a couple feet from getting back on the road. I just wonder if maybe, there's something under the car that this affected? There's nothing I found. No kinked exhaust, nothing broken, or obvious, etc...

Anyways today I did a compression test. All 4 tested out very close. 170 psi on 1 and 3, and 165-ish on 2 and 4. Granted, these numbers might be skewed high due to using an old pressure gauge. But none the less, they're all the same which is the main thing. Spose I could do an actual leak-down, but I see no need, all considering.

Leaving the battery unplugged over night. I'm sure that will do nothing other than annoy me with having to put up with goofy shifting for a while. lol
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Old 05-13-2009, 09:58 AM   #5
Bluefoton
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...lol at checking the TB for bird's nests

The three things I can thing of are a) wheel alignment is off after your little tango on the road, b) your fuel station suddenly started selling crap gas and adds10% ethanol, and c) your O2 sensor starts to fail.
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Old 05-13-2009, 11:15 AM   #6
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Quote:
Originally Posted by Bluefoton View Post
...lol at checking the TB for bird's nests

The three things I can thing of are a) wheel alignment is off after your little tango on the road, b) your fuel station suddenly started selling crap gas and adds10% ethanol, and c) your O2 sensor starts to fail.
I agree... check wheel alignment and you might want to think about replacing the O2 sensors. A fresh oil change with good synthetic oil might help (I recommend 0W-30). Also, make sure your tires are at the recommended inflation level.

My latest set of tires has a maximum inflation rating on the sidewall of 44 psi, so I keep 'em at 40 psi. This gives me an extra MPG or two.

With the car on a flat surface, does it seem to roll easily? If you can get it rolling easily by just pushing it, you don't have anything dragging.

Good luck! I like getting good gas mileage, too.

--Bob
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Old 05-13-2009, 12:31 PM   #7
dalehatestheriaa
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change gas stations, +1 on the alignment.
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Old 05-13-2009, 12:36 PM   #8
FlightService
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+1 on O2 sensors and +1 on changing gas stations.

Also the cat might be clogging up
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Old 05-13-2009, 12:52 PM   #9
aberg12012
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All good suggestions... Thanks! Its funny how i had thought of them all myself but never really gave them any merrit till i go to some forum and have some one else mention them too. I guess one could call it forumitis?

As for bird nests in the TB... I once bought an old camry from a friend with a sticky throttle. Like a moron i didnt even think about it for a few hundred miles till one day i got fed up with it enough.... As soon as i pulled off the air box and saw the chewed up filter i turned white with embarrasment, then purple when i exposed the TB and saw the mess that the valves had slowly been munching on. I called up my friend that day to let him know i found where all that missing dog food he was storing next to the car had gone... Car still ran forever of coarse!
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Old 05-13-2009, 03:18 PM   #10
lanz0r
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Reset the ecu? :P worth a try
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Old 05-13-2009, 04:30 PM   #11
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Quote:
Originally Posted by chazly413 View Post
Try cleaning the MAF.
IIRC, that's a MAP mill.
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Old 05-13-2009, 04:39 PM   #12
bobko
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Quote:
Originally Posted by aberg12012 View Post
As for bird nests in the TB... I once bought an old camry from a friend with a sticky throttle. Like a moron i didnt even think about it for a few hundred miles till one day i got fed up with it enough.... As soon as i pulled off the air box and saw the chewed up filter i turned white with embarrasment, then purple when i exposed the TB and saw the mess that the valves had slowly been munching on. I called up my friend that day to let him know i found where all that missing dog food he was storing next to the car had gone... Car still ran forever of course!
I had a family of mice living in the trunk of my 2003 Legacy sedan and they somehow managed to avoid getting caught in the four sticky traps I put back there. I even added a dab of peanut butter to the middle of each trap to make them more appealing but it only made my trunk smell like lunchtime in 3rd grade. I finally found the mother nursing eight babies and was able to scoop them up and carry them out of the garage. The little buggers can do a lot of damage, then stink up the car if one dies.

--Bob
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Old 05-13-2009, 06:44 PM   #13
chazly413
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Quote:
Originally Posted by LetItSnow View Post
IIRC, that's a MAP mill.
Haha, didn't even see his info.
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Old 05-14-2009, 01:41 AM   #14
aberg12012
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MAF... No worries. I knew better.

Whats really stinky is when a full grown fox snake makes its way deep into your ventilation system to hybernate for the winter... Freezes, then thaws in the spring. And rather than tearing the car appart, you just run the heater all summer to accelerate the rate of decomposition so it stops stinking... Asap. Dont ask... Lol
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