[Botnik]

As far as the hot spot theory goes, metal happens to be an incredibly good heat conductor, and any hot spots will readily share their heat with the rest of the metal it is in contact with.

Metal conducts heat well, but heat moves slowly. You can solder one end of something while holding the other end. (Assuming you are proficient at soldering.) Metal conducts heat faster than a heat insulator, but the heat isn't exactly zipping through there, it takes time to heat things up, it takes time to add the thermal energy to the metal, and the hot spots will be present.

If you are heating one end of a piece of metal, by idling an engine, then when it reaches steady state, you will have a heat gradient, with the heated end at one temperature, and the other end being cooler. Then when you start driving, presumably the heat you put into one end is going to heat up that end, and it again will take some time for the heat to travel through, and the other end will remain at the "idle" temperature until the heat conducts over.

It gets more complex when your piece of metal has a bunch of passages through it and a thermostat controls how much fluid passes through, etc.

 

[Botnik]

Try sweating solder into a copper pipe joint, or one that is already soldered to come apart that has water, or a similar boiling point liquid in it.
It is basically impossible.

Yup, can’t do it with typical soldering tools. And the engine has a lot of fluid in it, right near the source of heat, that at the right temperature is also circulated away into other equipment that transfers the heat into a large supply of another fluid (air).

 

[Botnik]

not going to sit and idle a modern, daily driven vehicle with engine management just to warm it up

You could idle it some seconds to recoat things with lube, so definitely start the engine first, before you put the belt on, adjust the radio etc, but this idling is not for thermal purposes.

 

[Botnik]

the impact of an extended idle

I haven't researched the theoretical or empirical about another idling cost, but have heard from mechanics that engines are designed to be at operating temperature, and also at operating load. I've heard that an idling engine has parts that are not (as much) under the main directions of force expected, and wear happens in different areas when idling. Again I haven't looked into it and am not an engine designer. But police cars etc idle a lot, and seem okay (between maintenance sessions), so maybe the main cost is fuel.

Re soldering, heating a small point on a wire is only one type of soldering. I haven't done this, but I suspect you could hold onto a copper pipe (or heatsink) and dip it into a pot of molten solder, and be done with your connection. It does take longer when using a torch, giving the heat time to move along. It's less common for people to have solder pots at home, but in a work environment you might have one.