What is the purpose of fast idle?

[irowiki]

So I tried googling this and only got fast idle in regards to diesels which is completely different. So I'll ask here!

What is the purpose of fast idle when a car is cold, especially a carbureted car?

Is it good to wait until it kicks down (several minutes) before driving when the car is cold? I usually wait about 2 minutes, manually kick down the idle, then drive off.

I ask because the stock setting of 3,000 RPM seems so high! My car was just screaming this morning, I need to get the tach out and check it (this is the 81 with no tach).

[marlinh]

Maybe this article will help shed some light.

http://www.hemmings.com/mus/stories/200 ... tips2.html

[irowiki]

I saw that but it didn't really explain the purpose of fast idle!

[Petros]

Fast idle is necessary during cold running. When the engine and induction system is cold not as much of the fuel vaporizes as it passes through the carb and into the combustion chamber, so it is difficult to get a reliable ignition (only the vaporized fuel will ignite, the droplets do not burn until they are vaporized during the combustion process). the choke makes it run rich to reduce the risk of stall out during cold running, but it can still have a tendency to stall out when cold, so speeding the engine up during cold running serves several purposes: it brakes up the fuel droplets better during induction to get more reliable ignition (due to the higher intake air speed and turbulence in the intake manifold), it helps over comes the higher drag of cold thick oil on the internal parts, and less risk of stalling out should a misfire occur during cold running, plus it helps speed up the warm-up.

You can back off the cold idle speed if your engine does not stall out at a lower speed, 3k seems a bit high to me. I usually like about 2200-2300 rpm for cold fast idle, but it does not usually get as cold here as it does where you live.

I usually only let mine warm for 30 sec to one min, giving it low short jabs at the throttle to speed the warm up (I wait until I see my oil pressure build up before I depress the throttle, I have a gauge installed). But I also start driving off very slowly and carefully with a very gentle throttle foot until I see the temp gauge come up off the throttle. Cold staring it and driving off in a big hurry will be particularly hard on, not just the bearings, but the extra rich fuel/air mixture also tends to wash the oil off the cylinder walls speeding up the wear on the piston rings. Once oil is splashed all over, and it warms up a bit (so the pistons are heated up and fit in their bores with minimal clearance), the cylinder walls are safe.

Supposedly waiting several min. to start driving prolongs the engine life, but it also means you consume much more fuel while not getting anywhere. so my solution is to allow short warm up, make sure oil pressure is up and engine idle speed stabilizes, and than drive off very gently on it, just barley above engine idle speeds.

[irowiki]

Thanks Peter!

Now if I jab the throttle during cold start, the fast idle cam is released and it goes down to more normal levels. (maybe the second step of the fast idle cam?). I get 3,000 RPM if I depress the throttle twice, then start the car without doing much else. It'll start low (1-1.5k) then after maybe 20 seconds, slowly rev up to 3,000 RPM. It stays there for 2-3 minutes until the TVSV starts moving, then the choke opener kicks it down.

The underhood sticker says 3,000 RPM so that's why I had set it at that, that was the main point of my question, as I wasn't sure the car needed to be sitting there screaming at 3,000 RPM for a few minutes (seems to warm up the car nice and fast). I'm going to try around 2,300 as per your suggestion. Unless you think I should just step it down right off so it fast idles at a lower cam step. I need to test and see how this works with a car with a Tach.

We've been getting into the 20's at night but we had one night where it was 12 degrees F. Lo and behold, the 81 I've worked hard on getting back to stock, started right up

[Devin]

I want to put in a hand throttle! Doubles as cruise control. (just kidding)