Just a few minutes ago a friend of mine mentioned this to me. Apparently he was surfing a message board (which he didn’t say) and came across this assertion - but rather more than that, it seemed to be widely accepted conventional wisdom.
So now the question: Is this true? Neither he nor I could think of a plausable reason, with one exception, why letting your car idle would be worse than driving it for the same period of time. The exception was that without forced air moving over the radiator and engine allowing the car to idle might tax the cooling system and allow an excessive temperature increase.
Any thoughts? On its face I think the idea seems silly, but what do I know?
While I don’t think that idling for MORE than 5 minutes has any benefit other than allowing your heater to be effective, there are a couple considerations for idling for at least a few minutes (assuming we’re talking about cold weather here).
1: To get the juices flowing (oil)
2: To allow the mixture to adjust and thus prevent stalling.
3: To allow expansion of metal.
The first and second are obvious enough, but why the third? Well, the cylinder heads, rings, and shafts will provide better compression (less seepage of combustion and thus cleaner/more efficient running).
There are several things that happen when you idle your car, but I don’t think they are important enough to cause any kind of damage.
Oxygen sensor takes longer to reach operational temperature.
Catalytic converter will take too long (if it does ever) to reach operational temp.
The engine is warmed up, but gearbox, CV joints, differentials etc are not.
As for temperature increase, I haven’t noticed any. Actually, I have noticed that the electric fan turns on for some time and then turns off again, which probably means that the engine temperature can be adequately controled while idling.
Your oil pressure will be very low when you are running at idle speed. When your oil pressure is low, the amount of oil being pumped around your heads is minimal. The less oil circulating, the more wear on the engine.
I never really thought about this until a bud of mine (ace mechanic) was working on an engine. He had the valve covers off and the engine was idling. No (or very little) oil was being pumped up to the top of the heads. If he would have pushed the accelerator, we would have been covered in oil.
Actually, I did consider the possibility that letting the car idle would reduce the oil and/or water circulation, but knowing nothing about cars I wasn’t sure whether this actually happened or not. I had always assumed that at idle the car pumps adequate oil to lubricate the engine and that increased flow at higher RPMs was primarily for keeping the oil temperature from getting too high and thus degrading the oil (and to a lesser extent for helping to cool the engine).
Not having seen the thread in question I can’t say with certainty, but my friend’s report suggests that the consensus was that letting the car idle, say, overnight was somehow REALLy bad. In other words, letting it idle was somehow worse than driving it all night. Is there any truth to this, and if so why is letting the car idle worse?
I can’t believe I actually found this Car Talk column that I recall reading in the newspaper eight years ago. Anyway, the Car Talk guys (who seem to know what they’re talking about) answered a guy who wrote:
Come to think of it, that was an old car we were working on, these days since cars are totally computerized, they probably compensate for the low oil pressure under idle conditions. One thing to think about if you are going to let your car idle for a long time, make sure there is adequate ventilation. I know (well, knew) a dude that passed out in his car while it was idling, he had a bad muffler that leaked, he had the windows rolled up in the car. Unfortunate outcome for that night.
I know on diesel engines that letting them idle will lower combustion chamber temps to the point that not all the diesel that is injected is burned and washes the oil off the cylinder walls and then dilutes the oil.
I’m not sure on gas engines but possibly more blow by gasses since the rings seal better the more pressure there is, increasing fuel dilution of the oil and adding acids. Depending on the car the cooling system may not be able to keep the engine cool without the added air flow over the radiator when the car is moving I know of some cars that have that problem.
Prolonged idling (such as the day and a half in the preceding post) probably will bring the cooling fan on and I don’t know whether or not they are intended for a 100% duty cycle. Other than that I don’t think it will hurt anything except your pocket book. Of course, some engines might possibly have a marginal oil pump.
In cold climates truckers let their trucks idle all night long while they get a night’s rest. Of course, diesel trucks have pretty robust cooling systems.
Air flow through the radiator is not the issue. Whether the fan is belt-driven or electric motor-driven, if the cooling system is working properly the fan will provide sufficient airflow and the car will not overheat.
Oil pressure is not the issue. If the engine and oil pump are in reasonably good shape, oil pressure at idle is more than sufficient for the engine’s no-load idling operation.
Most likely the concern is unburnt fuel diluting the oil and a somewhat more rapid production of harmful byproducts, a la dieselmech’s post. This would be more of an issue with a carbureted car, where fuel management is much cruder than on modern fuel injection systems with sophisticated electronic controls.
Thanks for the replies everyone. I think that pretty well sets the matter at rest, as far as it goes.
I would like to modify the question, though. Are there any reasons why this might have been a problem in the past? Perhaps due to oil dilution in carbeurated engines as dieselmech and Gary T have suggested? Any other reasons this might have been a problem in older cars?
In short, I wonder how this idea gained such seemingly wide currency. Perhaps it was once true, or at least a useful piece of popular wisdom, and now circulates even though it isn’t valid anymore.
[Note: To be fair, maybe this isn’t widely believed. Has anyone here besides me ever heard of this before? I don’t mean to blow it out of proportion, but I am curious.]
First, the Car Talk guys often don’t really know what their talking about. Their primary function is entertainment. When I occasionally happen to hear their show, I’m frequently annoyed at the poor quality of the “advice” given. They like to say things in jest that the typical car-ignorant caller might mistake as actual advice.
B, while prolonged idling isn’t going to destroy your engine immediately, it’s definitely a Bad Thing. The engine pollutes more, dirties it’s oil faster, and wastes a buttload of fuel, among other things.
I seem to remember hearing that older cars with breaker points and condensers, as opposed to modern ignition systems, were much more likely to foul the spark plugs when left idling. I can’t really be more specific as my experience is mostly limited to more modern vehicles without such archaic technology
This certainly isn’t a concern anymore though, and even at the time would only require a new set of spark plugs. Not exactly a big deal…
Surlely this can not be that much of a problem. If it was why would truckers leave their trucks running when they sleep etc? I listen to the CB all the time and always hear complaints of how much it costs to change oil, gas prices. Why, if it does ruin the oil would they continue to run the engine? That would in the end cost more money then it would be worth. I can understand it it was really hot or cold to keep the heater running, but truckers seem to do it all the time and for hours on end.
Actually, I wasn’t quoting their radio show, but their archived newspaper column.
And while they make a lot of jokes on the radio and in print, their serious advice (which, IMHO, is pretty easy to distinguish from the jokes) generally seems to be sound.
As far as factual information goes, they appear to be dead on. They’ve correctly addressed many UL about cars and driving.
They have pretty good credentials, too, including the fact that both are MIT grads. (The older brother actually turned down Harvard to attend MIT.)
Because they don’t read there owners manual. It’s not going to cause imediate engine damage, it’ll just cause it to wear out faster. Also it most states it’s illegal to idle a diesel engine for extended amounts of time normally under 30 minutes it’s not strongly enforced in most areas.
They will up the rpm above idle to help keep the cylinder temps up. With the electronics now some manafactures allow the company to set limits on idle times. Some have systems that will up the engine speed and selevtivly shut down cylinders so a 6 cylinder engine would run on 3 for awhile at around double the idle speed the switch to the other 3 cylinders for awhile to keep wear even. I have seen some trucks with a small generator to power the heater and AC systems but that is an expensive option.
