Should you allow a diesel engine to warm up before driving off?
No. You should depart immediately, but slowly. This is true for both diesel and petrol engines.
Idling a cold engine and waiting to warm-up before departing reduces the engine life. Why? Precisely because the engine shouldn’t run cold. Let me explain.
When idling at, say, 750 rpm, the engine warms up very slowly, especially if the engine is diesel. It will take, say, 15 minutes of idling to warm up. 15 × 750 = 11,250 revolutions. This means that all of the moving components made 11,250 cold, component-wearing revolutions.
If you depart immediately but slowly and drive at, say, 1500 rpm, the engine will reach proper temperature much sooner, after only, say, 5 minutes. 5 × 1500 = 7,500 revolutions. This means that all of the moving components made only 7,500 cold, component-wearing revolutions.
If you depart immediately and aggressively, the engine will heat-up even sooner, making even less revolutions cold, but the load on the components will offset the quick warm-up benefits.
Counter-intuitively, the colder the weather, the more damaging idle warming is: the already long idle warm-up time is even longer. So, in winter, it is even more important to depart immediately but slowly.
So, if we we order the start-up styles from the most damaging to the least damaging:
1. Cold engine, aggressive start (most damaging)
2. Cold engine, waiting to warm-up while idling (moderately damaging)
3. Cold engine, immediate slow departure (least damaging).
I understand this is counter-intuitive (many things about cars are!), but if you really think about it makes perfect sense.