2008 Impreza 2.5 gas mileage down 40 percent overnight
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This is a discussion on 2008 Impreza 2.5 gas mileage down 40 percent overnight within the Tuning: Electronic Engine Management forums, part of the Tech & Modifying & General Repairs category; My gas mileage stated by the computer is down almost 40 percent overnight. I also noticed when the ignition key ...

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    2008 Impreza 2.5 gas mileage down 40 percent overnight

    My gas mileage stated by the computer is down almost 40 percent overnight. I also noticed when the ignition key is on and the engine is off there is a faint squeal that comes from the top of the engine towards the firewall. I suspect an electronics, emissions or fuel richness issue. Any clues what to do to get back my MPG is greatly appreciated. The plugs have 50k miles on them but the car runs smooth with no missing. I have gone through a tank of gas and refilled at a different station no mileage improvement. Two weeks ago the car got 24 MPG around town, now it is 15 MPG at best. Thanks in advance for your help!

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    MAINEiac 11blackSTi's Avatar
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    Have you manually calculated your MPG to verify that the computer reading is accurate?

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    Registered User Ingo's Avatar
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    If there's no other indicators - just keep driving for a little while longer, a CEL should come up and help with diagnosis. My WRX started acting up, running sick, rough idle with occasional stall, CEL came on and I was on my way... Actually I had the CEL first, reset, seemed OK, then got sick and then CEL came back. A noise that you didn't notice before may not be anything. There's a bunch of things making noise, you just normally overlook that. Once there is some kind of trouble - that's when you start paying attention.
    You can't have everything - where would you put it?

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    OMG WTF MainFrame's Avatar
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    Are you just gauging it by what the computer says? Because it is likely wrong. Divide the number of miles driven by the amount of gas it takes to fill up, report back.

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    Quote Originally Posted by 11blackSTi View Post
    Have you manually calculated your MPG to verify that the computer reading is accurate?
    I just refilled and the odo miles since last fill / this purchase = 18.401 MPG
    The computer read 19.5 since last fill. A 6 percent error rate higher. The computer is typically about 2 percent high but this was only a 119 mile sample.

    Anyhow, MPG is not good. "There's somthin happnin here but I don't know what it is." Bob Dylan

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    OMG WTF MainFrame's Avatar
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    It wouldn't be bad to pull your learning view and see what closed loop corrections you have going on.. I wouldn't expect a failing front O2 sensor already on an 08, but a look at learning view would give you an indication of what's going on. If you had a wideband O2 sensor installed that would be even more helpful.

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    Registered User Ingo's Avatar
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    Does the wideband replace front or back sensor or is it stand-alone just for a separate gauge?
    You can't have everything - where would you put it?

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    OMG WTF MainFrame's Avatar
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    Separate sensor, separate location, separate gauge.. unless you have an 02-05 WRX, then you can chuck the rear O2 sensor and install your wideband sensor there, like I did.

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    Quote Originally Posted by MainFrame View Post
    It wouldn't be bad to pull your learning view and see what closed loop corrections you have going on.. I wouldn't expect a failing front O2 sensor already on an 08, but a look at learning view would give you an indication of what's going on. If you had a wideband O2 sensor installed that would be even more helpful.
    "It wouldn't be bad to pull your learning view and see what closed loop corrections you have going on."
    Sorry, I am a newbie at this. I can build and fix computers, but I have spent little time on modern vehicles. I could though at one time set your points gap or change your generator or voltage regulator, but them daze is long gone.
    How does one pull the "learning view?"
    Thanks!

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    Registered User Ingo's Avatar
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    You'll WANT to spend some money on your car: a Tactrix adapter and some PD software. Check the search function and search for "Romraider", "tactrix" Open source (-tuning), Learning View. Own a Laptop, pretty much required for these...
    There is also Cobb App, way more expensive, but easier to use.
    Learning view is software that reads out the ECU's "learned" changes to fueling and timing... It helps a lot with finding out what's going on. This stuff also works as your code reader, tuning tool, logging tool, etc...
    You can't have everything - where would you put it?

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    Quote Originally Posted by Ingo View Post
    You'll WANT to spend some money on your car: a Tactrix adapter and some PD software. Check the search function and search for "Romraider", "tactrix" Open source (-tuning), Learning View. Own a Laptop, pretty much required for these...
    There is also Cobb App, way more expensive, but easier to use.
    Learning view is software that reads out the ECU's "learned" changes to fueling and timing... It helps a lot with finding out what's going on. This stuff also works as your code reader, tuning tool, logging tool, etc...
    Thank you! I'll check it out.

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    Registered User Formula156's Avatar
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    Road and wind conditions are extremely important, for you to get accurate mpg what you need to do fill up your gas tank and reset your mpg to zero. and drive normally that will allow your vehicle to calculate mpg correctly and you sould do this every time you fill up your tank.
    Divide the miles traveled by the amount of gallons it took to refill the tank.

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    MAINEiac 11blackSTi's Avatar
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    Another thing to check is your air filter, tire pressure (also, your alignment could be off), possibly the fuel injectors need to be cleaned (although that probably wouldn't be a problem that would show up overnight, but worth looking into). To be honest, 18ish MPG around town really isn't that bad though.

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    Quote Originally Posted by 11blackSTi View Post
    Another thing to check is your air filter, tire pressure (also, your alignment could be off), possibly the fuel injectors need to be cleaned (although that probably wouldn't be a problem that would show up overnight, but worth looking into). To be honest, 18ish MPG around town really isn't that bad though.
    All good points....

    I checked the air filter. It was pretty clean but I did order (Amazon) the K&N cleaner/oiling kit – the filter is about 30k miles old. I knocked out the large chunks of debris and put it back. I’ll do the job right when the stuff comes in.

    As for the chip reader path I’m looking at $180 for hardware plus the learning curve and still would have old plugs. I should probably just get a tune up. Plugs appear to be a pain in the arse to replace on these boxers.

    Sadly, I am beginning to suspect the new tires I just put on it even though they brag about gas mileage. I also sanded the shine off the rotors and put a set of new pads at the same time. The original pads had 51k on them and still had life so I am not hard on brakes. The pads are not binding on the rotors. I got the fronts up off the ground and they turn freely by hand. So that ain't it.

    I put these tires on the car:
    Primacy MXV4 | Michelin Tires

    And took off these at 51k miles and still had likely 5k miles left.
    Bridgestone Potenza RE970AS Pole Position - Free Delivery Available | TireBuyer.com

    The new tires are at 35 psi. The door calls for 33.5 psi I believe. So they are plenty aired up.

    My journey for a cause continues…..

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    OMG WTF MainFrame's Avatar
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    Shouldn't use an oiled air filter, it can dirty your MAF. .which I suppose could also be a problem. Wouldn't hurt to clean it.

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