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Thread: Why do cars idle high when cold?

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    Why do cars idle high when cold?

    I've noticed that my car (and my mother's as well as my friend's) idle higher when they are cold than when they are hot. The difference is about 400rpms it seems. I was wondering if that is something that the ECU does in order to warm the engine up quicker or there are some other factors (such as cold oil creating more drag). Also what are your thoughts on warming up the engine before you set off?

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    I guess thats a feature to warm it up more quickly

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    With a carb, it will idle high because the choke engages when its cold to ease the starting process by dumping more fuel into the cylinders.
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    Injection engines have the same cold-start phase, you just don't have to push a button anymore or pump the gas pedal.

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    Master of Disaster Steve Levin's Avatar
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    It's to overcome the higher internal friction of various components when the car is cold.

    Steve

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    Quote Originally Posted by Crazyjeeper View Post
    With a carb, it will idle high because the choke engages when its cold to ease the starting process by dumping more fuel into the cylinders.
    Quote Originally Posted by DarkReaper View Post
    Injection engines have the same cold-start phase, you just don't have to push a button anymore or pump the gas pedal.
    Quote Originally Posted by Steve Levin View Post
    It's to overcome the higher internal friction of various components when the car is cold.

    Steve
    Also, I read somewhere that a cold engine messes up the atomization of the fuel (the fuel liquefies when it contact with the cold metal instead of misting) and therefore you need to put more fuel to keep it from stalling.
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    Its supposed to help reduce hydrocarbon emissions on start up by bringing the operating temp up quicker
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    this brings me to another valid question

    why do cars with warm engines in the winter make that water smoke out of the exhaust when idling and not when running? I know it IS water what you see, but why only in idle? Is it because the exhaust is hotter when revving?
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    That's cause (as it's been told to me) the engine gets hot enough and the exhaust gasses should be invisible... the water is visible when idling cos it's been sitting for a while and built up some what, that doesn;t happen when running an engine that's continuously hot enough to prevent any build up.
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    Makes sense....
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    O.K basically, yes it is to overcome greater internal friction when the engine is cold and the lower amount of vaporization of fuel when the motor is cold....Oh btw this is only on a normal port fuel injected spark petrol engine. Diesels etc, no idea, however I belive they are the same.

    These graphs are a pain in the arse for tuners two create, because if you put too much fuel in you'll foul the plugs or too little and it won't run toooooo crash hot. One reason I refuse to become a tuner...lol The little tweak at the end of the IAC map is to assist in hot starting...

    These two graphs where dumped off my race car engines computer, you'll notice when the motor is cold, the IAC (Idle Air Control) motor is "more open" and the air fuel ratio is around 12:1 which is on the rich side

    Technically these two graphs are then compared to the base idle map (on GM based EFI systems) before being actually used, but it shows what is basically going on. The computer these maps come from is a GM Delco 808 EFI computer from the Mid 80's. Wonderful little things. Not all GM's stuff is crap, EFI computers is one are where they really kick arse...well at least compared to other Japanese or Alfa Romeo computers I have to deal with

    As far as emissions go when the poor 'lil motor is cold: They are S**t. Since we are dumping in bulk fuel to get the motor to function the AFR is wrong so the CAT won't function and the unburnt hydrocarbons and CO are terrible.

    EDit:

    Also the CAT converter converts unburnt hydrocarbons into CO2 and water, which would explain the presence of (some) water in your exhaust.

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    Quote Originally Posted by Blayde View Post
    That's cause (as it's been told to me) the engine gets hot enough and the exhaust gasses should be invisible... the water is visible when idling cos it's been sitting for a while and built up some what, that doesn;t happen when running an engine that's continuously hot enough to prevent any build up.
    And the amount of gases flowing from the exhaust of an idling engine is significantly lower than that of an engine at higher revs. Less gas cools down quicker, lower temp means condensing water, means visible steam fumes.
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