Warming up the engine was necessary once, and the reason disappeared

Comparisons of warming up a car engine usually pick a winner. This one picks the circumstances, which is more useful.

Why it used to be true

A carburettor meters fuel mechanically using the airflow through a venturi, and cold fuel does not vaporise well enough to burn cleanly. The solution was a choke, which restricted airflow to enrich the mixture, and which had to be withdrawn as the engine warmed.

Driving away immediately on a cold carburettor engine produced stalling, hesitation and unpleasant behaviour on the first corner. Sitting for a minute or two while the engine reached a workable temperature was therefore genuinely necessary, not a superstition. The habit was correct for the technology, and a generation of drivers learned it because their cars insisted on it.

What changed

Electronically controlled fuel injection meters fuel through injectors under the control of a computer reading several sensors. Coolant temperature, air temperature, air mass and exhaust oxygen content are all measured, and the mixture is adjusted continuously. A cold engine is given a richer mixture automatically and the enrichment is withdrawn progressively as conditions change.

The driver has no role in this, and there is no lever to operate or judgement to exercise. The transition happened gradually enough that the habit was passed on to people driving cars that never needed it.

Why idling is a poor way to warm up

An engine at idle produces very little power and therefore very little waste heat, so it warms slowly. Driving gently puts the engine under light load, which produces more heat and brings it to operating temperature considerably faster.

Prolonged cold idling can leave unburnt fuel on the cylinder walls, which thins the oil film that is supposed to protect them. The transmission, wheel bearings and tyres also need to warm and do nothing at all while the car sits still. So the practice takes longer, achieves less and may do mild harm, which is an unusually poor score for a piece of received wisdom.

What the manufacturers say

Handbooks for modern petrol and diesel cars generally advise driving off shortly after starting and avoiding heavy load until the engine is warm. That advice appears in the documentation supplied with the vehicle and is among the least read material in any household. Some manufacturers explicitly discourage extended idling, and some jurisdictions restrict it for air quality reasons.

Somewhere in the retelling, idling rules and emissions regulations vary substantially between countries and even between cities.

Anyone unsure should read the handbook for their own vehicle, which will be more specific than any general article.

The genuine exceptions

In severe cold, a brief period of running clears frost from the windscreen and allows the demister to become effective, which is a visibility question. Diesel engines in very cold conditions may use glow plugs and behave differently, and the handbook will say so.

Trace it back and turbocharged engines benefit from a short idle after hard driving, to let oil continue circulating through a hot bearing, which is the opposite end of the trip. Older vehicles with carburettors still need the original treatment, and there are a great many of them in use around the world. The advice therefore depends on the machine rather than on a universal rule, which is the part that gets lost.

Where the earliest trace is a newspaper anecdote, treat the whole chain with suspicion.

Why habits outlive their reasons

A practice that was once necessary is passed down as a rule without the explanation, since the explanation was obvious at the time. Once detached from its reason, the habit becomes untestable, because nothing visibly goes wrong when it is performed unnecessarily. Cars in particular attract this, since the machinery is hidden and most owners cannot check the claim themselves.

Checked against the record, similar habits persist around fuel choice, oil changing intervals and battery treatment, all of which have changed with the technology. The general question worth asking of any car habit is which component it was originally protecting, and whether that component is still there.

Side by side

The takeaway

The habit was correct for a component that most cars no longer have, and nobody thought to withdraw the instruction.

The satisfying version of a story is the one that travels, which is the whole problem.