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If I sit in my car for 30-40 minutes with the AC and the car idling, does that "hurt" the engine, or does it simply waste gas and pollute the air? Would it be better if I turned off the car? A similar question was asked, but it never addressed non-diesel engines.

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7 Answers 7

up vote 11 down vote accepted

letting the car idle isn't good for the car, especially with the AC on. What you're basically accomplishing is aiding in the car wearing without racking up any miles. With the cylinders firing, you are using several electrical components, putting some small amount of stress on the belts, etc.

If you've ever seen ads for cars claiming high mileage but declaring that they are "highway miles" the claim they're attempting to make is that they aren't doing city driving with the vehicle (unnecessary idling and as such effectively putting miles on the engine but not the odometer).

Whenever the engine is running, you are slowly wearing out some of the components of the vehicle. Doing this for a few minutes here or there doesn't make for a huge difference, but 30-40 minutes at a time would be more than equal to the amount of time you'd spend idling at lights/in traffic in a major city during a 1+hour commute.

In short, 2-3 minutes is fine; anything over 10, and the general consensus is that it's a bad idea.

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You aren't really giving any reasons why idling would be any worse than time spent driving. It's like saying "driving the car isn't good for the car" –  qes Aug 25 '11 at 20:59
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That is functionally true. Driving your car wears out parts of your car. Driving in rougher conditions (towing, winter driving, extreme heat, offroading, etc.) is worse for your car, as the additional stress will wear parts out faster. Letting your car idle isn't actively bad for the drive train, but it certainly does put strain on the engine, additionally engaging the AC will put the engine under load, and work the engine harder even though you're not driving anywhere. Just having the cylinders firing and the belts moving is inducing wear on those parts. –  Jessie Aug 26 '11 at 13:10
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I would expect that running the engine at idle (no load except its own internal friction and accessories on the belts) puts a lot less wear on it than running it under load. "City miles" are not just "worse" in that there's wear that's not reflected on the odometer, but also in terms of load starting from a stop, varying throttle (with moments of suboptimal gas/air mix), etc. So I don't think this answer really settles the issue of how "bad for the engine" idling might be. –  R.. Dec 14 '13 at 6:51

Aside from financial and environmental considerations, U.S. and Canadian governmental agencies say that idling produces various negative effects on engine components.

During idling engine does not work at its most efficient mode, and the fuel combustion is incomplete. That leads to glazing of combustion chamber and reducing effectiveness of spark plugs in petrol engines (plugs with heavy deposits can foul, and that can reduce fuel consumption by 4 to 5%). Also petrol can contaminate the motor oil which reduces its lubricating qualities (that could mean more frequent oil changes and/or increased wear on the engine).

Also, as the engine runs at less than ideal operating temperature, which causes water vapour (a product of combustion) to accumulate in exhaust system (at proper temperature and at higher speed of exiting gasses caused by increased RPMs it clears out pretty good), which leads to corrosion. Also, idling produces up to 10–12% more emissions than driving for the same amount of petrol used due to incomplete combustion and ineffective operation of catalytic converter.

As for people worrying about excessive wear on starting system components, here is some math. According to some estimates, frequent re-starting of the engine can cost about $10 a year extra in replacement parts etc. (In my experience, I had never had to replace a starter (or solenoid, or flywheel gears etc.) in many years I owned several cars (I do drive less than most). And even if people treat their vehicles like junk it might be once in 8–10 years.).

Now, let us assume we drive mid-sized car that averages 25 MPG for a 1000 miles (~1600km) a month, or 12000 mi/year. That makes it use about 480 gal/year (12000/25). Assuming modest reduction in fuel efficiency of 5%, that amounts to 24 extra gal/year for the same amount of miles driven ((480 x 1.05) - 480). At low estimate of $3.50 per gallon, that amounts to $84 a year in increased fuel consumption. Saving that, new starter would pay off in less than three years.

Those are very, very conservative calculations. Some numbers cited by various no-idle campaigns estimate that 10 minutes of idling uses about 0.1 gallon of fuel (for a small car). If you idle 10 minutes a day, it adds up to more than 35 gallons a year. And it only takes 10-15 seconds worth of fuel to restart the car. You can save real money just by shutting off the engine on rail-road crossings, at drive-throughs, on long street lights (not recommended for average person) etc.

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Anecdotally, I've seen a number of (and been in one) car(s) that has overheated while idling. Without airflow, you're depending on the fans and thermostat for more than if you were driving. If all's well with the car, you should be perfectly fine idling for some time. However, if the cooling system has an issue, or if the oil pump is getting weak, you may end up with a breakage that otherwise would not have happened (or at least wouldn't have happened as soon). Think of extended idling as a stress test for the car, no problem all is well, but if not, you'll find out about it! :-)

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You have a much higher probability of not having enough oil pressure at idle. If the engine has any issues - such as wear causing tolerances to be larger than initially engineered, or too thin of oil - the lack of oil pressure will be more exaggerated. Idling for long periods could be disastrously more damaging to your engine then the same amount of time spent cruising.

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Engine wearing happens mostly when the engine old is cold or the engine is over heating. When the temp is cold pumped engine oil doesn't flow well through block drillings and heads causing the engine components to wear. Now in overheating situation the engine oil is thinner and can't protected engine components as well, certain parts will melt and would cause a water pump casket break.

Over heating is a result of over stressing the engine or having not efficient radiator. Warm engine oil provides the best protection for engine components.

To answer your question having the AC ON while idling will hurt the engine if the engine can't maintain a recommended operating temperature, no cold weather can stop an engine from over heating without a radiator system. So yes, idling with AC will hurt your engine but only if your vehicle doesn't have an efficient radiator system, such as an older model vehicles. Not sure if I can name vehicles models here without being dragged to court.

Over heating is basically what would cause the engine to wear quicker. If you can avoid that, then doesn't matter if the car is running at highway speed or idling.

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On a completely different note... in SOME places (I live in one such place, Notably Vermont), it's ILLEGAL to leave the keys in an unattended vehicle, or to leave an unattended vehicle idling. True, we Vermonters do that a lot in wintertime - leave the vehicle running while we dash into a grocery store so the steering wheel isn't covered in frost when we return - but it IS illegal, and it IS prosecutable.

What happens if someone jumps into your vehicle and makes off with it? You'd likely be found nearly as culpable as the thief. A running car is a blatant invitation to theft.

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I live in Canada and I can tell you the wear on starting an engine is greater than idling for a few minutes, guaranteed, especially since here in the winter it can only take 20-30 minutes for your engine to drop from full operating temp down to room temperature if it isn't running, and cold starting an engine means less viscous oil in the crankcase as well as little to no oil on the valvetrain during the first rotation or two. Additionally, all engines apply more fuel during warm up than during normal operational temperatures, simply to help them warm up faster. You should NEVER drive a cold engine (any time of year), get it to at least 75% of full operating temp before driving it, A: because you are going to be running a rich fuel/air mix during warm up and B: Metals (especially aluminum which most modern 4 cylinders are made of this) does not like to heat up fast, you increase the risk of metal warpage, I've seen people blow head gaskets by hammering a cold engine in the dead of winter and warping the head, that isn't a 10$ fix... that's a 1000$ fix if you get a shop to do it.

The rule of thumb I use personally is everytime I start driving a new vehicle I first time how long it takes the engine to warm up from cold, I then memorize that time and if I'm going to be stopping anywhere for LONGER than that time I will turn my engine off, lets say it takes 10 minutes to warm it up {at -40f it takes more like 25, but lets say 10} and I'm going to run into a gas station to grab something, if I'm going to be in and out in under 10 minutes, I will leave the engine going (with the doors locked mind you, spare keys and keyless entry are pretty much a must have for that) but if I'm going into a friends house for 15 minutes, I'll shut the engine off.

This is a rough guide, you want your engine hotter for high RPM driving sprints (highway trips) to help the heat transfer to your transmission and exterior components so they don't heat up too quickly either.

Theres a hell of a lot more I could say about this topic, but I'll end it here, I'm hungry.

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