what tells the engine to rev high when cold?

[Erin]

Any car I have ever had – once the outside temp gets below freezing, the engine will rev up a couple grand for a short bit.
You now what am talking about, you go out to start it in the morning, you hear the pulleys making their respective noises,
smoke screen from the exhaust, and the engine idles pretty high for a bit.

Yet, this only happens when a car sits “overnight”. Even a couple hours later, it starts and almost immediately assumes normal idle.

I am curious then – what info does the engine use to idle real high like that at first start after a freezing night?

Topic

[college man]

The coolant temp sensor. cts

[Mike]

It’s the coolant temp sensor like college man said, but there’s a bit more to it. When cold a little extra fuel is injected because the mixture needs to be a little richer when the combustion chambers are physically cold. To a small extent adding fuel can increase RPM just like letting a little extra air in by cracking the throttle plate. If you were to adjust the idle mixture on a pre-OBD or carbureted car, the effect can be seen first hand. Once the coolant temp sensor shows the computer it’s just a few degrees warmer, it assumes the combustion chambers have warmed up enough and begins cutting back the extra fuel and the idle starts dropping as the fuel injection amount is reduced to normal..

[Lorrin Barth]

A cold engine when started is not as stable as a warm engine. The idle speed needs to be sufficient to allow the work to occur the engine is doing. But you say, what work is it doing at idle? Well, you just discharged the battery somewhat starting the engine so the generator may be working hard. The oil may be thick and hard to pump. In the cold people turn on window defrosters, heater motors and headlights. Also, gasoline engines burn only vapor and gasoline does not like to be vapor in the cold. Much extra gasoline is injected to obtain the correct mixture. This too can make the engine unstable. The solution to all of this is to increase the idle speed until the engine warms.

So, as above it is the engine computer sampling the coolant temperature by way of a sensor and then opening the stepper motor in the throttle body to let in more air, and also in some older vehicles a thermostat type device that opens inside the throttle body to let in more air.

[Author]

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