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RPMs -- I'm curious about something

NOT having any problem, merely curious about something I’ve noticed for years about car engine rpms. Using my current 2014 Camry as an example.

On a cold engine start the rpms are about 2000 then slowly drop to about 1000 while idling as I’m fastening the seatbelt. By the time I drive a few blocks idle rpms at a stop are slightly less than 1000. By the time I drive about a third of a mile idle rpms at a stop are about 750.

Starting the engine once even partially warmed up, say about a mile driving, the idle rpms drop much faster from the 2000 down to a consistent 750.

So why the slow drop to a slightly higher idle rpm on a cold engine versus the rapid drop to a lower idle rpm when the car has been driven only a short distance and not fully warmed up or when fully warmed up?

Just curious.

What are you curious about /

@Renegade Sorry, I accidently hit the post key before writing out my question. Typo fumble finger typing on my phone. I’m just talented. Sorry about that.

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No problem I have the same problem big finger’s and small button’s to mash. As to your question I don’t know I have a 1/4 mile long downhill driveway that is level at the house when I start my truck cold the rpm’s go to 1100 I move about 30 feet then coast down the driveway about 1/4 to half way down it drop’s to 950 rpm’s and every time I stop anywhere at a stop sign or light it stay’s a 950 rpm’s or if I shut it of after it is warmed it stay’ af 950.

More engine speed as well as more fuel will prevent a cold engine from stalling. That is a basic answer but the engine is supposed to do that. In our earlier days, this was done with mechanical stuff attached to the carburetor, and it related to the position of the choke. When the engine got slightly warmer you could hit the gas pedal and the idle speed would go lower. Sometimes there were three positions with three speeds. The speeds were provided by the fast idle cam that the idle adjustment screw would rest on. After the third step would move out of the way the idle screw would rest on the basic setting. It was fun to adjust that stuff. When you pressed the gas pedal to squirt fuel into the carburetor, the fast idle stuff would spring into position because the idle adjustment screw would get out of the way and then rest on the fast idle cam when you took your foot off the gas pedal to start the engine.

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Part of it is to keep the engine running, another part of it is to get catalytic converter(s) up to temperature quickly, so that they start working as soon as practical.

One of the PCM inputs used to regulate idle rpm is coolant temperature. A cold engine needs a faster idle than a warm engine to keep from stalling. This is identical to how carbureted car with an automatic choke and a fast idle cam would behave.

Engines run most efficiently when warmed up . So when it is completely cold running the rpm higher temporarily gets the engine and the catalytic converters warmed up faster and then it will start to slow the idle down to a normal 550-700 rpm depending on your car and engine. Here is a pretty good explanation for the technically minded into what goes into all this. https://ricksfreeautorepairadvice.com/open-loop-versus-closed-loop/

Thank you everyone. All good information that helps me understand how the engine works. Among other things, after reading the linked article, for the first time I get the distinction between open loop and closed loop in computer function. And I now understand why a cold engine is more prone to stalling than a warmed engine. Now it also makes sense to me why computerized fuel injection engines are less likely to stall on a cold start than carburated engines.

Thank you for indulging my curiosity.

Marnet
…still reading, still learning!

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And back before we had loops, the two main reasons for a fast idle were that cold oil puts more drag on the engine, and the cold intake manifold causes the fuel air mixture to condense on it’s surfaces. Faster idle moves the mixture faster=less chance for it to condense.

I had assumed that my rpm question was tied to engine oil but had in my thinking mistakenly thought it the primary factor rather than the fuel / air mix being the primary factor.

If you want some more insight into this issue, Google a now obsolete device called a “heat riser valve”. It’s not just interesting that cars of the past ran as well as they did, it’s amazing they ran at all!

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I’ll bet if you pay real close attention to when the transmission upshifts when the engine is still cold vs. when it is at full operating temp you will find that the upshifts happen later when the engine is cold. Higher Rpm means a faster warmup.

Yes, I have noticed that about shifting in every car I’ve driven.

I’ve only had a tachometer in my cars the last thirteen of the forty-one years I’ve been driving. So I’ve always paid attention to the sound and feel of engine rpms in driving and still do by habit rather than rely on the tach gauge. For example, in situations when an engine seems straining to reach a shift point on an uphill I ease off the gas a tiny fraction for a second or speed up a bit more for a second, depending on traffic, to let the transmission shift without strain.

Paying attention to the sound, feel, and performance of the engine was one of the lessons I was taught when learning to drive until it became ingrained second nature to me. I’ve often thought I would have handled a manual well had I ever gotten the chance to learn to drive a stick shift. But at this point in life I am highly content to stick with driving automatics. (pun included at no charge)