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Mileage, Scan Gauge

Discussion in 'Technical Chat' started by mcgiiver, Aug 11, 2009.

  1. Aug 11, 2009 at 7:10 AM
    #1
    mcgiiver

    mcgiiver [OP] Well-Known Member

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    Some people say they shift to neutral and coast to save on gas. On the '09 4 cyl's, wouldn't it be better to coast in gear, becasue when in gear, above 1000 rpm, and clutch up, the fuel flow to the engine is completely shut off as I understand the system. If coasting in neutral, fuel is still flowing to keep the engine idling.
    Maybe someone with more tech knowledge than me can shed some light on this.
    Maybe someone with a Scan Guage can check it out assuming a Scan Gauge measures instantaneous fuel flow, which I am not sure.
     
  2. Aug 11, 2009 at 7:12 AM
    #2
    Brunes

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    Coasting in neutral is illegal in many places...I don't know the benefits of gas savings from doing it...but it does put you in danger of not being able to control your car completely should the need arise.
     
  3. Aug 11, 2009 at 7:17 AM
    #3
    mcgiiver

    mcgiiver [OP] Well-Known Member

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    , but that wasn't the question.
     
  4. Aug 11, 2009 at 7:21 AM
    #4
    Dark Knight

    Dark Knight Well-Known Member

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    think about it...if fuel was compleatly shut off...so is your engine
     
  5. Aug 11, 2009 at 7:23 AM
    #5
    fletch aka

    fletch aka www.BeLikeBrit.org

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  6. Aug 11, 2009 at 7:35 AM
    #6
    lasllc

    lasllc Well-Known Member

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    Scangage wont help much on this as it displays TPS as "number", might be throttle pecent open or might not be, it is simply a referance based on the trucks computer signal representing throttle opening and is set up out of the box at 22 or so I think; consequently , your IMG with foot off the throttle and coasting displays what Scangage is calculating based on the TPS you have set up (or not) not, necessairly, what the engine is actually doing; the default setting is good for most cars (it says) but it has no idea what the Tacoma, specifically, is doing.

    I know, I know, commen sense says that if you are coasting and the Scangage says 99.99 MPG the throttle is probably shut, but you can not assume that as it is a computer and we dont have access to the code. You dont know how Scangage is interprating the cars computer to calculate that 99.9 mpg - exactly when does toacoma completely shut the throttle body down? who knows ? Smog control comes into play here also.

    For example, when I installed mine i set the TPS cut off at 17 so that my AMG would incorporate the distance travelled while coasting, on the assumption that the throttle is still open a bit for the smog system to flush out the vapors etc and do whtever job it needs to do while coasting down. my Scangage does not display an IMG of 85 or more untill I am below approx 5 MPH.
     
  7. Aug 11, 2009 at 7:36 AM
    #7
    Brunes

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    IMO it does answer the question- I think what the fuel does doesn't matter...I want to be safe and in control of my truck...so I'll leave it in gear...no matter what a scanguage finds out.

    This makes sense
     
  8. Aug 11, 2009 at 8:07 AM
    #8
    Snipe

    Snipe Well-Known Member

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    I don't coast down every hill I come too, but when the right opportunity presents itself, yeah I will do it.

    Going down a hill and in gear my scangage will show somewhere around 60 MPG and as soon as I pop it out of gear it will jump up to around 80 or better.
     
  9. Aug 11, 2009 at 9:38 AM
    #9
    mcgiiver

    mcgiiver [OP] Well-Known Member

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    Thank you, this helps answer my question. My question had nothing to do with safety of coasting in neutral. I never even said I coast in neutral.
     
  10. Aug 11, 2009 at 3:49 PM
    #10
    spasur

    spasur Well-Known Member

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    not necessarily...assuming you are still in gear and all parts of your truck function normally as they should, the energy stored by your truck moving would keep the engine rotating as long as the engine speed is above idle. You can test this (I am not telling you to do this) in theory by driving down an empty road or a parking lot in 2nd or 3rd and shut off your engine...it will keep spinning, and you may need to change your shorts depending on the situation...I found this out by accidentally bumping the key with my knee once. It was not a tacoma.

    However, I have no idea if the ECM cuts fuel entirely when coasting, I would assume it keeps sending some fuel to keep the engine running smooth and in case you push in the clutch or apply the throttle. That little bit of fuel might be enough to give you the change in mpg that Snipe reported and therefore would be greater than the fuel needed to idle.
     
  11. Aug 11, 2009 at 3:54 PM
    #11
    Janster

    Janster Old & Forgetful

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    You're splitten hairs.....

    Should've bought a hybrid if you were that concerned about gas.
     
  12. Aug 11, 2009 at 5:02 PM
    #12
    mcgiiver

    mcgiiver [OP] Well-Known Member

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    The ECM can cut off the fuel entirely, but I am not sure it does. On my 1997 2.4 engine, in deceleration, in gear,the ECM would cut off the fuel entirely unless certain things happened like depressing the clutch (that's why there is a clutch switch), engine rpms drop below 1000 (ECM thinks the engine wants to idle or you put your foot back on the gas pedal. You cannot even tell the fuel is cut off, because your in gear the engine doesn't stall, but it needs no fuel whatsoever to continue to turn. As said the energy in the moving truck keeps it spinning.

    I suspect Snipe is correct in his observation that coasting in gear uses more fuel than coasting in neutral for the reason that the manufacturer wants the exhaust slightly rich during decel for emissions reasons.

    In either case we are talking miniscule amounts of fuel compared to that used in driving mode.
     
  13. Aug 11, 2009 at 5:31 PM
    #13
    williegator

    williegator Well-Known Member

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    My scan gauge goes to 9999 MPG when coasting to a stoplight. Maybbe it's a 2009 4.0 thing. I did read either on here or another site that this is exaclty what the engine is doing, shut down the injectors....Oh and the GPH goes to 0.0 at the same time.
     
  14. Aug 11, 2009 at 6:03 PM
    #14
    HBMurphy

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  15. Aug 11, 2009 at 6:13 PM
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    HBMurphy

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  16. Dec 11, 2009 at 7:39 PM
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    Isthatahemi

    Isthatahemi Well-Known Member

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    Above approximatly 1000 rpms, when you let off the throttle, a number of things happen. One of those things is the fuel is cut. The fuel injectors stop cyling, and do not inject fuel. On the auto V6, it's almost instant. On the 4 cyl manual, it takes a second or two. So the short answer is leave it in gear......To idle the engine in neutral consumes more fuel, (in this case). And it is idling, no mater how fast the vehicle is moving.....
     
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