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I've noticed that my 2008 Jetta 2.5L 5W40 generally starts up with an idle RPM of at most 1.2K RPM around the freezing temperature (say when the temp sensor reads 33°F), or even at just about 1K RPM when it's even colder.

However, in the warm temperatures (when the temp sensor reads 40°F (4°C) or above), it's always starting up with a 1.6K RPM idle, no exceptions, unless the engine is already warm.

When the engine is warm, the RPM is generally around 600 to 700 RPM when idle.

Is it normal for the starting idle RPM to be much lower in the winter? Do I need to wait before driving the car?

In the non-freezing weather, I generally wait until it goes down from 1600 to about 1000 RPM before driving, which generally takes maybe half a minute or so, but what kind of indication could be relied upon in the winter, when the idle RPM is already under 1000 RPM almost right from the start?

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4 Answers 4

I am from Norway so I am sorry if I use wrong words or misspell.

It is normal that the starting rpm is dropping when it's cold. The battery is struggling a bit to deliver power needed to turn the starter. When it's cold the engine shrinks a little and makes is harder to start.

I will recommend you to start the engine and then let it run for a couple of minutes.

I live in Norway so at wintertime when I start my car, I'll always wait from 1 to 5 minutes before I drive.

I don't know what's you're definition of cold, but my is below -15 degrees Celsius

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Thanks for joining the site. I think, however, your answer does not really answer the question. The OP is suggesting it is idling slower after start, not about the starting process itself, and then also, specific to the 2008 Jetta 2.5L. –  Paulster2 Feb 11 at 12:36
    
Thanks for the welcome! OP is asking about the starting rpm, not idling. And if he should wait before driving. My answer to that is that the battery can't deliver the same amount of power to the starter, and the starter is requiring more power to turn the engine when it is cold. Therefore the starting rpm drops. –  Super Feb 11 at 12:58
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@Super The listed RPMs don't match with the actual starting RPMs so I'm inclined to think it's the initial/cold idle. Cranking RPMs are usually quite low (250-500 or so). –  Brian Knoblauch Feb 11 at 13:05

On my diesel truck, idling RPM on cold days is lower and have to use the hand throttle to keep the RPM high enough to not stall. As for the reason behind this, my guess is thicker oil and fluids make it harder to turn the engine.

As for the waiting before driving the car, I personally run the engine on idle for less than a minute and gently start to drive to save the mechanical parts that are still froze. Read what Natural Resources Canada as to say about vehicle warm-up.

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That's the opposite of what my cars do. Warm days they start at 1.2-1.5k before idling down. On cold days they start at 2.2-2.5k (which is so high I can't even get them into gear until they settle down).

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From a mechanical point of view, if the engine would be at the same settings as when warm, the cold engine would have lower rpm (and stall/not start), so: yes.

But cars nowadays have a "cold start device" that will raise the rpm after a cold start until the engine is warm. This will offset the "naturally lowered" cold start rpm and can range from not completely compensating the lowered rpm to overcompensating depending on the type of cold start device and on the programming of the cold start electronics. So in that respect the answer is: no.

The behaviour you observe with a recent car like yours where the engine behaviour is controlled electronically has little to do with what you can expect from a engine mechanical point of view and all to do with how the engineers who developed the machine decided it should behave in a cold start with low ambient temperature.

The first stage of this cold start procedure are the first few seconds until the engine is warm enough not to stall (that's what in ancient times the choke was for, or for diesel engines the "earlier injection"* or the "raised minimum rpm" cable). For gasoline cars, you have then a longer period where the minimum rpm is raised (over the warm "proper" idling which you quote for your car at 600 - 700 rpm) because the convertor and λ probe need to be warmed up asap so they start to work, even if you do not drive but idle. The minimum rpm you observe during this period is still not the proper idle rpm. It is an (electronically) raised rpm in order to make the engine warm up faster.
Whether this raised rpm is faster or slower with colder ambient temperature primarily depends on what the engine development specified how this raising should be programmed (e.g. whether the specification goes for an increased injected volume or for a fixed rpm. You could (should?) re-program it in another way.

Do I need to wait before driving the car?

If everything is as it should be, there should not be any need of waiting more than maybe few seconds (maybe 1 - 3, but 10 would be long even with my old diesel from the pre-electronic era which of late coughs a bit on one cylinder when it is cold).
If there's a bit of trouble with the engine, I'd wait to be sure it doesn't stall at the next crossing when you step off the gas.

This sounds like a description of the second stage (which in fact may be several stages) of the warming up:

I generally wait until it goes down from 1600 to about 1000 RPM before driving, which generally takes maybe half a minute or so

Warming up by idling (the 600 rpm idling) is not only bad for the environment, but also for the engine. It is generally recommended to warm up the engine at moderate rpm and power, i.e. by driving in a civilized fashion. My guess is that the raised rpm (over the warm engine idling rpm) is just the cold start device making sure the car is warming up fast even though you refuse to start driving.
In other words, the engineer designing your engine knew that people would leave the cold engine idling and had an increased rpm programmed for the warm up phase of the engine for one or two purposes

  • environmental norms have to be met which nowadays include the emission after cold start, and
  • this is also better for the engine.

* The mechanical "earlier injection" for diesel engines does not raise the rpm. As the injection is nowadays electronically controlled it would easily be possible to adjust the injection likewise. I do not know whether there exist engines where this is done without raising the minimum rpm. So I cannot even answer that the minumum rpm immediately after a cold start is always higher.

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Nice info, but, unfortunately, this doesn't at all address the question -- even the parts you quote are taken completely out of context. –  cnst Feb 16 at 0:35
    
@cnst: clearer now? I think your question is even for the specific engine not really answerable because it depends on the programming / electronic version of the cold start device. What context would you need for an answer of no (whether with or without explanation) to the question whether you need to wait before driving? –  cbeleites Feb 16 at 2:13
    
I tend to agree with @cnst here ... Not sure how your answer applies to the question. You talk about general idle at cold start, but not the specifics of the question at large. –  Paulster2 Feb 16 at 3:06
    
Sigh. Do I have to edit the question again? @cbeleites, your answer is still missing that I'm asking about "freezing" start, not "cold" start. And also that my engine's "freezing" start has specifically a noticeable lower RPM than its "cold" start, and not the other way around. So, again, you provide interesting info, but it's still mostly unrelated to the question (you now even suggest that I told you that I warm up my engine at the 600rpm idle, which I never do -- I only wait until it goes from 1.6k to 1.0k rpm prior to driving, it'll take a very long time to warm it up to 600rpm idle). –  cnst Feb 16 at 3:23

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