With modern combustion engines it is common advice that idling before driving away for more than a few seconds (to build up oil pressure) does more harm than good, even in "arctic" winter conditions (see linked article). The reasons given are that on idle the engine will be running with sub-optimal lubrication (cold oil at low pressure), with a rich mixture that further hinders lubrication and will eventually dilute the oil. Furthermore, due to the very low load, the engine will take a long time to warm up. One can also read that long idling damages the turbo-compressor.
On the other hand, the manuals of current Citroën cars say:
In very severe wintry conditions (temperature below -23 °C), to ensure the correct operation and durability of the mechanical components of your vehicle (engine and gearbox), leave the engine running for 4 minutes before moving off.
Of course, practically speaking, at such low temperatures one will be simply unable to drive off before the windshield has been warmed up somewhat. But let’s just consider the theory here: what’s the "optimal" duration of idling an engine before driving off? It is unlikely 4 minutes below -23 °C, and a few seconds otherwise. What about a more common -5 °C, say?
(I do know that engine heaters exist, but this is not the point here.)
And, here's a twist:
Does the situation change when the car has been parked on a high mountain with a long descent? Think of skiing or winter mountaineering, where the car has been parked at -10 °C all day, and there’s an immediate steep descent (6%-8% average) over an altitude difference of 1300 m, say.
When using the engine brake reasonably on such a descent, a gasoline engine will rev up to 3000 or 3500 1/min. I know from own experience that the engine warms up very slowly under such conditions, it is not yet fully warmed up at the end of such a descent - after all no fuel is injected when coasting.
Some people say that high revs are very bad for a cold engine, and this is clearly the case here.
Others say it is OK, because the load on the engine during coasting is low and it is properly lubricated (oil pressure is high, and no fuel is injected). Common experience seems to support this view: there are hundreds of thousands of people driving their cars down from mountain villages and ski resorts every day.
