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Thread: IDLE Throttle after depressing brakes.

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    Senior Member mitsumi's Avatar
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    Question IDLE Throttle after depressing brakes.

    Hi There,

    Q: Is it normal that when im on idle/parked and i pressed then depressed the breaks the throttle or rpm goes up then it goes back down after few seconds?

    Using Manual hatch 2015.


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        click to view fuel log View my fuel log 2015 Mirage GLS 1.2 manual: 3,108.4 mpg (US) ... 1,321.5 km/L ... 0.1 L/100 km ... 3,733.0 mpg (Imp)


  2. #2
    Still Plays With Cars Loren's Avatar
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    In short, yes.

    Power brakes operate by engine vacuum. There is a "vacuum reservoir" involved, so it's not a 100% direct connection to the intake manifold. But, yes, when you release the brake pedal, it's pulling vacuum from the engine to replenish the reservoir, and that can affect the idle because the MAP (manifold pressure) sensor is one of the inputs that the ECU uses to control the engine.

    Our engines are especially sensitive to things like this because they are what they are. They are a 3-cylinder engine idling at something like 650 rpm, which is pretty amazing in itself. Think about your typical 4-cylinder engine from the 90's that would idle at 850 rpm. The Mirage not only has fewer cylinders to fire (which should indicate a HIGHER idle speed to idle smoothly), but it's idling SLOWER. On top of that, it's engineered and tuned to be as efficient as possible, so it's trying to burn as LITTLE fuel as possible at idle.

    So, the slightest disruption in anything will cause a little hiccup, which the ECU will instantly adjust to by changing the idle air (which, I believe, is done using the DBW throttle on this car), fuel, ignition timing advance, or cam timing. It's going to say "hey, whoa, something's not right, stalling isn't cool, I need to take action", then bump the idle speed up a little to "regain control", and then it's going to say "oh, I'm okay", and settle back into a normal idle.

    I sit in awe at how well this engine idles and does absolutely amazing things at low RPM without a care in the world. Perhaps you have to be old enough to have spent time with some crappy carburetted 4-cylinder cars with no electronics to really appreciate it.

    Sorry for the ramble. But, yes, it's normal.
    Simplify and add lightness.

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    Moderator Eggman's Avatar
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    I figured it was related to electrical load, such as lighting up the brake lights. Is your AC running?
    2015 Mitsubishi Mirage ES 5-speed with pushbutton start, automatic climate controls, original Dunlop Enasaves and no center console.

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        click to view fuel log View my fuel log 2015 Mirage ES 1.2 manual: 49.6 mpg (US) ... 21.1 km/L ... 4.7 L/100 km ... 59.5 mpg (Imp)


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    Senior Member Dirk Diggler's Avatar
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    Without a doubt the 3A92 is tough little ba@tard. I really feels it purrrs @ 3krpm.

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    Quote Originally Posted by Loren View Post
    I sit in awe at how well this engine idles and does absolutely amazing things at low RPM without a care in the world. Perhaps you have to be old enough to have spent time with some crappy carburetted 4-cylinder cars with no electronics to really appreciate it.

    Sorry for the ramble. But, yes, it's normal.
    I don't miss carburetors on small cars at all!!!!

    My ex-wife's Plymouth Horizon - any time it got really cold (-20F), the carburetor would freeze wide open (full throttle)! Never really got corrected. Inspired our first fuel injected car purchase, however.

    My first car 1978 Honda Civic Wagon CVCC - I really liked that car, except for the carburetor issues. Local small town mechanic really messed it up, & it cost me some big bucks to have a Honda dealer correct it all. Those were my college years when funds were in short supply! It's the only car that I owned that still had a hand choke on the dash.

    I am old enough to appreciate fuel injection!

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    Senior Member Fummins's Avatar
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    As Loren already said, yes. Pump the brakes while stopped and the it'll idle up and down a lot.


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        click to view fuel log View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)


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