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This is a discussion on Mass Airflow Sensor within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Originally Posted by gfx If you wanna clear a CEL, disconnecting the battery, pressing the brake pedal until the odometer ...

  1. #16
    Moderating U! KurleeDaddeeWRX's Avatar
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    Quote Originally Posted by gfx
    If you wanna clear a CEL, disconnecting the battery, pressing the brake pedal until the odometer disappears, then connecting it again can usually make it go away. I sometimes get a CEL in the cold cold weather, and that makes it go away.
    That is one way of clearing a CEL. But if you disconnect the battery to reset the ECU, the first tank of gas will be swallowed up quick by the ECU relearning your driving style.

    The proper way of clearing a CEL is actually getting the CEL read to make sure it isn't something seriously wrong going on and then clearing it with a OBDII reader. And that way it won't affect your gas mileage which can be pretty expensive in these times.

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    Mass Airflow Sensor

    If your Mass Airflow Sensor failed, your car probably wouldn't be driveable period. I had this happen on a '84 Volvo and the car would start up, back-fire like crazy, and the exhaust system and catalytic converter would get extremely hot. I think the cat was even glowing cherry red!!
    Yes the part cost was about $500 CDN at the time, but the car was 15 yrs old so it didn't owe me anything. Drove it for another 5 yrs after that and sold it. Its still on the road. The lady that bought it uses it to commute to work 30 miles each way - in Canada through salt, slush snow and ice 4 months a year!!

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    The big problem with failing MAFs is that they do not fail completely in one go. The MAF in the WRX works out how much air is flowing by working out what current is required to keep the wire hot - the more air flowing - the more air is passing over it (regardless of air density) at altitude there are less air molecules and therefore do not cool as much.

    Anyhow - when gunk gets on the MAF wire - it insulates the wire and it does not need so much current to keep it warm - it reports that there is less air coming through and therefore the ECU puts in less fuel - which means it will actually run lean (bad).

    This can over time (as the gunk happens slowly) lead to holes burning in pistons and high EGTs

    Check your MAF regularly!!

    Cheers,

    Andrew

  5. #19
    gfx
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    Quote Originally Posted by KurleeDaddeeWRX
    That is one way of clearing a CEL. But if you disconnect the battery to reset the ECU, the first tank of gas will be swallowed up quick by the ECU relearning your driving style.

    The proper way of clearing a CEL is actually getting the CEL read to make sure it isn't something seriously wrong going on and then clearing it with a OBDII reader. And that way it won't affect your gas mileage which can be pretty expensive in these times.
    I always just use Shiv's method of bringing the multiplier back up. It relearns in 10 seconds
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    Moderating U! KurleeDaddeeWRX's Avatar
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    Quote Originally Posted by gfx
    I always just use Shiv's method of bringing the multiplier back up. It relearns in 10 seconds
    Exactly what I did when I had my WRX. But not everyone knows how to do this. That is why I posted the correct procedure for the newbs.

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    Shiv's Method

    What is the Shiv's method of bringing the multiplier back up and where is it posted?

    Thanks,

    ~r

    Warning: newbie learning area ahead

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    Quote Originally Posted by Gearboy
    What is the Shiv's method of bringing the multiplier back up and where is it posted?

    Thanks,

    ~r

    Warning: newbie learning area ahead
    Even though you are a newb, the SEARCH button works as good for us as it does for you. Here's a freebie

    Re-Mapped ECU Learning Trick 10-20hp in 5 seconds :)

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    Cool. Thanks.

    Next time I'll use the search engine.

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    Just ordered the EQUUS 3100 code reader. Expensive, but better than trusting the dealership, from what I've read.

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    should have just got an accessport if you use econ mode it pays for itself in a period of time and you can read and clear codes.

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    Quote Originally Posted by GIRex
    should have just got an accessport if you use econ mode it pays for itself in a period of time and you can read and clear codes.
    Yeah. Maybe so. But it's a lot more expensive and I'm not inclined to fool around with the factory settings due to the possibility of decreasing the longevity of the engine, which has already got 60,000-mi on it.

    Instead, I'm just going to keep it stock until I can afford to trade it in on either an STI or the new Legacy GT. Which ever car I get, I'll be doing my modding to it.

    ~~~~~~~~~~~~~~~~

    Just got back from the Subaru Dealership in Bakersfield. After most of the day and $160 worth of testing, I finally found out that the CEL was from a P0171 = system too lean. The shop manager showed me the workshop manual for dealing with this. In short, I'd have to pay a lot more labor to be absolutely sure that it's the MAF, but it probably is. There's a slight possibility that it could also be the fuel pressure sensor - or both.

    Since I've already removed and cleaned the MAF numerous times now (following directions found at ClubWRX), only to have the CEL pop back on after a few hundred miles, I decided to pop for a new MAF. But guess what. They didn't have one and have to order it! <Insert words of exasperation here> Couldn't afford to pop for a fuel pressure sensor too - especially without knowing for sure what's wrong.

  13. #27
    Registered User GIRex's Avatar
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    why dont you ask them to show you the code if you didnt trust them, i mean hell you paid for it. or just go to autozone where they do it for free.

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    Quote Originally Posted by GIRex
    why dont you ask them to show you the code if you didnt trust them, i mean hell you paid for it. or just go to autozone where they do it for free.
    I guess my last wasn't too clear. It's not that I distrust that the code is actually P0171, it's just that this code is a bit vague:

    "Fuel trim malfunction (A/F too lean)."

    There are a number of things that can cause this. I looked at the workshop manual. There is a whole list of diagnostic tests to do to try and narrow down the possibilities. Many of these tests were run today. That's how they got it down to the MAF and/or the fuel pressure sensor. In order to pin it down more, I'd have to pay for more diagnostic testing. So as you can see, it's more than just knowing what the code is in this case.

    ~~~~~~~~~~~~~~

    Here's the topper: after I had been home for a few hours (back from Bakersfield), we decided to go out to eat. One of our cars has a flat, one is still blocked by a pile of snow, and so that left the WRX. When I started it up...no CEL. Go figure. I've fixed nothing and yet, no CEL. No CEL all of the way to the restaurant. Ate. Came out and started it up an hour late...no CEL. Drove it home. No CEL. Very mysterious. But good.

  15. #29
    Registered User GIRex's Avatar
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    so are you planning on replacing the parts yourself?? and then reading the code to see if it works. Thats probably way cheaper than letting the dealership do it. I used to work at one and my wife still does so i get great discounts but its still pretty expensive.

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    Quote Originally Posted by GIRex
    so are you planning on replacing the parts yourself?? and then reading the code to see if it works. Thats probably way cheaper than letting the dealership do it. I used to work at one and my wife still does so i get great discounts but its still pretty expensive.
    I'm going to replace the MAF myself (not a big deal) and not replace the fuel pressure sensor yet. If no CEL pops up, I'm not going to worry about it. If a CEL pops up after I replace the MAF, then I'll have my OBDII code reader to see if we're getting the same code or something else - and go from there.

    I started up the Rex this morning and still no CEL. I replaced the oiled filter with a stock paper filter, just in case. Im heading out of town tomorrow and am planning on taking the Rex. Im taking break cleaner and a baggie just in case.

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