You're over-thinking this, or at least you're over-concerned. Police cars can run at idle for hours at a crash scene. While they're designed for that, it can be hard on an engine that has a standard cooling system. Some parts of the engine are lubricated by oil that splashes around or is sprayed from pressurized ports. That oiling is less effective at real low speeds. Engines can survive that less-efficient oiling longer than the parts that need pressurized oil, but over long periods of time that will increase wear.
Some engine parts get hotter as speed increases because more heat is generated but it is still carried away at a rate that is not related to speed. When engine speed decreases, those parts will cool a little. The issue with temperature is that parts expand as they heat up. Pistons in particular are "cam ground", meaning they're oval instead of round. Due to the way heat builds up and is dissipated, they become round when they're at proper operating temperature. That's why 99 percent of engine wear takes place in the first few minutes when the engine is cold, and almost no wear takes place during highway driving, even on long trips.
The goal of varying engine speed is to meet a variety of conditions so all mating parts have a chance to wear together. When they are allowed to form matching wear patterns early in their life, there tends to be very little additional wear later on. That's also why it is customary for a mechanic to reinstall those parts, when removed for repairs, with their previous mating parts.
Basically what your manual is saying is to drive the vehicle like you normally would. If you have to let it idle for ten minutes while waiting at a train crossing, that's nothing to be concerned about. If you have to drive for an hour at highway speed to get to the next town, do it. If it's normal day-to-day driving, that's fine. You want to avoid idling for a real long time, like a half hour, because you need the airflow through the radiator to keep the engine cool rather than waiting for it to get hot enough for an electric radiator fan to turn on. You want to avoid driving only at highway speed so parts can develop mating wear patterns when they contract due to cooler internal temperatures.
Most manufacturers no longer specify detailed break-in procedures. Metal alloys are better, oil has better additives that reduce wear, and parts are machined so precisely that very little additional wear needs to take place for them to develop nice mating wear surfaces.
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Thursday, July 3rd, 2014 AT 8:33 PM
