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I have an 08 ford focus that I haven't been driving. I just turn it on and let it run for about 30 minutes every few weeks so that it's not just sitting. One time I left it running in the parking lot and forgot about it until 14 hours later. How bad is this? What kind of damage could I expect from this?

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    Did it run out of fuel, or did you discover it and turn it off before it ran out? – Eric Hauenstein Aug 31 at 20:35
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    Just curious, how much gas did that use? – Jason Goemaat Aug 31 at 21:17
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    it didn't run out of fuel surprisingly. It only burned through about 5 gallons. – Mike Aug 31 at 22:22
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    Aside - this doesn't help your tyres avoid flat spots, nor does it help the gearbox or clutch. For some vehicles that depend on splash lubrication in odd places like swivels, there won't be any. Best drive for about 20 minutes. You don't have to get out of the car elsewhere. – Criggie Sep 1 at 3:57
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    very importantly: if someone does the same but in a garage, or any enclosed area (and this can happen also with a badly running furnace, etc) : abe.iastate.edu/extension-and-outreach/… – Olivier Dulac Sep 1 at 8:18
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There's probably no damage at all, the engine just ran at low revs for a long time. One issue you may have is if it ran out of fuel the line may have sucked sediment from the bottom of the fuel tank and drawn it into the fuel filter, which can clog the filter up. If you notice it's lagging a bit in the future what's what I'd look at. Fortunately fuel filter replacements are cheap. Also, some fuel pumps are designed to sit in the fuel tank and be cooled by the fuel, running very low on fuel could overheat the fuel pump, this could also lead to insufficient fuel pressure, especially at higher revs, however it's unlikely a single instance would cause that much damage.

Incidentally, just letting your car idle isn't good enough, you need to actually drive the car around to keep everything working and the tires circular. A 20 minute drive once per week will keep the battery charged and everything working.

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    Great answer. Funny, look at the drive cycle of Police cars. In some cases those engines run essentially 24x7, often for long periods of time at engine idle. – zipzit Aug 31 at 9:23
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    @zipzit Police cars at least used to have things like better alternators and more cooling in order to support that use case. (Agree once isn't going to hurt anything much, but to do it all the time like police cars do--better build for it.) auto.howstuffworks.com/police-car2.htm – user3067860 Aug 31 at 16:15
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    Why do people always go on about "when the fuel in the tank is low it might mean the fuel pickup pipe sucks sediment up" - where do they think the fuel pickup pipe is the rest of the time (when the fuel is not low) ? – Caius Jard Sep 1 at 11:55
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    So suction is some kind of mystical force that spreads throughout a liquid, and when there is less liquid, the suction is less spread out so you can pack more suction in per cubic inch, and it does a better job of picking up sediment? Does that even sound believable to you? People who assert this should look at how a fuel tank is designed, and how modern fuel injection system works – Caius Jard Sep 1 at 16:28
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    Sounds like a process that affects cars constantly, especially those driving around/slopping their tank contents around in a continual shaking - so why aren't the streets littered with broken down cars everywhere, blocked fuel filters abound. If a tank contains half a pound of crap, it contains half a pound of crap, and if its suspended in the fuel it will eventually make its way up the pickup pipe and into the filter. The volume of fuel in the tank is immaterial to this process. I also disagree with your assertion that a blocked filter filters worse than an unblocked one. – Caius Jard Sep 1 at 17:17
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I have an 08 ford focus that I haven't been driving. I just turn it on and let it run for about 30 minutes every few weeks so that it's not just sitting.

Don't. Idling an engine is the worst way to warm it up. Also, if you do this because you want to keep the battery charged, the charger may not be able to produce as big current when idling. It's good to let all parts including tires move, so instead of idling it, please consider driving it for the equivalent time. The increased costs due to fuel consumption is more than outweighed by the reduced risk that someone steals the car.

How bad is this?

It's bad. Two risks can happen:

  • The car can get stolen.
  • If the cooling system is not up to the task, the car can overheat -- especially if the fan doesn't work. Normally the fan is not needed due to air current as the car is moving, but when idling at standstill it's a different game.

What kind of damage could I expect from this?

If it didn't get stolen and didn't overheat, you're probably fine. As GdD explained, it's not recommended to run out of fuel, but you didn't mention running out of fuel.

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    Assuming you have a second key and lock the doors while it's idling, the increased risk of theft is not all that high. – R.. GitHub STOP HELPING ICE Aug 31 at 17:47
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    Alternators are designed to put out plenty of power to charge your car at idle speeds. You're only going to run into issues charging at idle if you're also powering a massive aftermarket sound system or other non-standard electronics. – Logarr Aug 31 at 19:09
  • @R..GitHubSTOPHELPINGICE Depends on the neighborhood, right? I can think of some neighborhoods where "just break a window and drive away" makes a difference. Maybe some cars, too, if you have one that has the fob transponder, etc., so it's a huge pain to drive it away without the key--but your windows are still easily breakable. (Plus you put a sign on your car saying the key is there, since people can hear it running.) – user3067860 Sep 1 at 16:35
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    Don't forget that (possibly in this case but definitely in a lot of cases) with wireless keys it's possible to walk away from the car having started it with the presence of the key, then leave it idling, locked with no key inside and the computer beeping away saying "key not present". It's still driveable, and if a thief steals it and stalls it they might have a problem, though some cars will allow restart of a stalled car within a certain timeout even if the key is absent.. – Caius Jard Sep 1 at 16:46
  • This. And don't forget about the toxic fumes that are the result of inefficient combustion. – scuro Sep 3 at 8:25
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Short response: No damage - idling a car does not hurt it.

Long response: That said, you shouldn't be idling it to warm it up/charge the battery/whatever. What I do with my cars in storage is put the battery on a tender. Then every two weeks or so, turn the key on to unlock the transmission (key on, not start), put it in neutral, and roll it a few inches. This fights flat spots and keeps the battery charged. If you can't hook up a tender (as you mentioned a parking lot), then I recommend keeping the battery somewhere with adequate airflow (such as a porch or patio) and keeping it on a tender there.

Keep in mind that most of the wear on an engine occurs when it's cold. So starting it just to move it a few inches isn't good. And idling it rather than driving it means that it stays cold for longer, thus extra wear. There are also other concerns if the engine doesn't heat up enough on a regular basis (when the car is started and doesn't get up to operating temp). Not starting it at all is the best way to 'preserve' it. I'd say drive it every 2-3 months to be safe, but I've gone 6 on mine without any issues.

Make sure you're still changing the oil as recommended even though it's being stored.

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    Well, change the oil once maybe.. But if you're gonna lay up a car for eg 5 years I don't see the point in changing the oil every year, when the engine is never even started – Caius Jard Sep 1 at 11:57
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Prolonged or repeated idling can cause coking and sooting of the cylinders and plugs. This can make it hard to start, taking it for workout for 30 minutes or so will clean that up.

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    From what century is the vehicle to which this answer relates? – Caius Jard Sep 1 at 11:57
  • @CaiusJard modern cars suffer, too. Esp. in cold weather. – fraxinus Sep 3 at 12:22
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No severe damage has been done to your engine except for the fact that due to idling for a long time, a little bit of carbon deposits have been developed inside your combustion chambers. You can remove them by driving your car and revving the engine a little bit to increase the pressure inside cylinders and burn unwanted deposits. Never let your car idle for more than a few minutes because during idling, combustion doesn't happen efficiently and blow-by is high because there is not enough pressure to seal the piston rings against cylinder walls. Too much blow-by will dilute your oil and diluted oil will not be able to lubricate your engine properly. Blow-by means the amount of combustion gasses and combustion byproducts that go past through the gap between rings and cylinder walls. They will enter into the oil sump.

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  • Could you please provide a credible source of evidence for your claim of "during idling ... blow-by is high because there is not enough pressure to seal the piston rings against cylinder walls"? – Caius Jard Sep 3 at 12:38
  • During idling, incomplete combustion happens and this is a major factor of oil dilution. Moreover, according to the following source, one supplier of blowby flow meters said contrary to what many people think an engine typically has more blowby at idle than at higher rpms. As the speed goes up, the rings actually seal better and blowby drops. Therefore, this source also refers to the fact that at lower engine speeds, ring sealing is less effective. Here is the source: aa1car.com/library/engine_blowby.htm – Nariman Asgharian Sep 3 at 13:01
  • So the bit where you asserted that it is cylinder pressure that causes a piston ring to seal against a cylinder wall is your own fabrication? – Caius Jard Sep 3 at 13:13
  • revving the engine doesn't increase pressure much. to increase pressure need to open throttle and not have the engine race. eg hill climbs in a steep gear, – Jasen Sep 3 at 13:41
  • Fabrication?! I don't know what you are up to but there are also lots of sources for this fact like this one which says: Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. courses.washington.edu/engr100/Section_Wei/engine/… – Nariman Asgharian Sep 3 at 14:16
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Idling a cold engine is bad for the engine itself and bad for the environment. The simple fact that the engine is not on its target operating temperature means that combustion is only partial and toxic fumes are emitted and the internal parts and oil are polluted.

Just like the others mentioned, you should drive the car. All parts need to move and tires need to roll.

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Consequences to the car: None. Consequences to people near, if it inside a garage or closed place: Asfixiation by carbon monoxide.

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