0

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.

  • 1
    In arctic conditions, there is often an engine block heater connected. Also, the answer may change based on whether it is a diesel engine, turbo, etc., as you noted. Which scenarios and engine configurations are of most interest here? – NL - Apologize to Monica Nov 13 at 15:42
  • 3
    @SolarMike “ Most modern petrol vehicles cut off the fuel supply completely when coasting (over-running) in gear” - en.wikipedia.org/wiki/Energy-efficient_driving – HandyHowie Nov 13 at 18:52
  • 3
    @SolarMike You said - “Can guarantee that driving down a mountain still uses fuel”. That is not true. – HandyHowie Nov 13 at 19:41
  • 2
    @Solar Mike - I also have a diesel automobile, but when I am engine braking down a steep hill without touching the throttle and without allowing engine speed to fall at or below idle set point, no fuel is being injected into the cylinders. It's not necessary; the engine is already being turned above idle by the grade and being in gear. – Tedwin Nov 13 at 20:59
  • 1
    @dwizum It is commonly accepted that many cold starts (=short distance driving) shorten the life span of an engine. Now I could imagine that driving down 15 miles at 7% slope with a cold engine in winter at 3000 revs is like a hundred cold starts or even worse. My question is: is it really so, and if yes, does idling before the descent help? How long should one idle: 1 minute or rather 10? If, on the contrary, long descents with a cold engine are "OK", I would like to know why this is so: cold engines are supposed to be driven gently (low load, no high revs). – spheniscus Nov 14 at 8:32
0

1 you have adequate oil pressure the instant the oil light goes out. So forget about the low oil pressure argument. It's baloney.

What oil doesn't have when cold is great flow since it's so thick. But idling the engine, contrary to popular believe DOES NOT warm the oil. It only warms the coolant. The fastest way to warm the oil is to drive it---but without flooring it. Be gently for the first few miles.

See this post for all the pros and cons of warming a modern engine.

  • Well, it does warm the oil, albeit much more slowly than if there was a load on the engine... – Tedwin Nov 14 at 22:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.