Today in a parking spot, I spotted a Volkswagen Golf 4 of which the engine was running and the air conditioner was turning at a high speed (hearable from outside) in order to cool down the car, but there was nobody inside.

I did not recognize whether a key was inserted or not. But had the owner left the car while no key is inserted, that could cause the car to get stolen easily, so that would be a moronic idea.

Maybe it was a car modified to keep the engine running somehow without a key inserted and with the doors locked.

Is this a common car modification, if any?

If so, is any of you familiar with this?

That could be very practical in summer, in order to keep the car's cabin cold while oneself is absent for a short time.

Running the car's engine for one hour with air conditioning enabled roughly consumes 1 litre/liter of fuel (0.26 gallons) per hour, which would be worth it.

  • 1
    Are you sure you werent hearing the radiator fan? Jul 26, 2019 at 15:13

5 Answers 5


In cold climates in is a "standard" modification to have a remote engine starter, so you can warm up the car before using it. That could obviously also be used to run the aircon in hot weather.

If someone has two sets of keys, they could lock the doors with the other key still in the ignition.

But in the UK it is illegal to leave any vehicle "unattended" on a public road with the engine running, and in fact it's illegal to run the engine of a parked vehicle for "a long time" even if the driver is present, unless you are working on the car.

Apart from air pollution and possibly excessive noise, stationary cars with the engine running do sometimes catch fire, or set fire to things underneath them, or get involved in accidents where they need to be moved, etc...

  • “stationary cars with the engine running do sometimes catch fire” – Which part of the car catches fire first?
    – neverMind9
    Jul 25, 2019 at 22:02
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    @neverMind9 one of the parts producing heat....
    – Solar Mike
    Jul 26, 2019 at 3:45
  • @SolarMike But doesn't the engine have a radiator that cools it down?
    – neverMind9
    Jul 26, 2019 at 7:11
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    @neverMind9 lots of systems, only one needs to fail or be interfered with by an owner with limited knowledge to cause an issue...
    – Solar Mike
    Jul 26, 2019 at 7:14
  • @SolarMike I see. But the same could happen during driving.
    – neverMind9
    Jul 26, 2019 at 7:15

I guess you were in southern US, not unusual I believe they leave a key in the ignition, lock the car and carry a second key. I see a few cars running each day; not necessarily for a short time. My 79 Olds 350 ( 5.7) carburetor burned 2 gal ( 8 L) /hour at idle , about the same as going 40 mph. Fuel injection has been a big improvement.

  • It was actually Germany. I have seen that for the first time.
    – neverMind9
    Jul 26, 2019 at 7:12

Don't do this.

First and foremost it's straight up illegal in Germany. No ifs, not buts. To quote (translated) -

In particular, it is prohibited to unnecessarily run vehicle engines

It's also illegal in the UK (sec. 42 of the Road Traffic Act), and in the following US States:

Arizona, California, Colorado, Connecticut, Delaware District of Columbia, Georgia, Hawaii, Illinois, Louisiana, Maine, Maryland, Massachusetts, Minnesota, Missouri, Nevada, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, South Carolina, Texas, Utah, Vermont, Virginia, Washington, , Wisconsin, and Wyoming

Secondly - you're adding unnecessary wear on the engine - I can't find the link off hand but if I recall correctly each hour of idling is equivalent of 33 miles worth of driving.

Thirdly you risk draining the battery - you're placing a relatively high load on the alternator under conditions where it is unable to provide optimal current (i.e. idle).

Fourthly it creates unnecessary emissions/pollution. I'm no tree-hugger (I do drive a V8 after all) but still there's an element of being reasonable here.

Finally you're running the car in hugely sub-optimal conditions for cooling - much of the performance of the cooling system relies on there being frequent airflow over the radiator. The radiator fans help when it's stationary but aren't able to match what the motion of the car even at modest speeds can achieve. Add in that you've got additional heat being dumped from the aircon rad (usually in close proximity to the main engine radiator) and you're asking a great deal of it. Especially on very hot days!

And if the intent is to do this when the car is unattended then you won't be around to deal with any overheating condition and you could come back to a wrecked engine that would have been easily averted were you present.

Yes people do do this, and quite frequently they "get away with it" and nothing bad happens - it doesn't make it any less of a dumb idea though.

For bonus stupidity if you're doing it with the keys-in-the-ignition method it's a spectacular theft risk. It doesn't matter if you've locked the door if someone throws a rock through it they can be in the car and driving down the road in seconds.

Getting into a hot car that's been baking in the sun all day sucks - I get it. But in the grand scheme of things it's a tiny inconvenience and you can easily use things like this to reduce the problem without incurring all the risks of running the engine in this manner.

  • Good point about breaking a window and stealing the car. That insurance claim would be interesting to say the least - "I left the car idling with the key in the ignition...".
    – Paul Lydon
    Jul 26, 2019 at 11:24
  • @PaulLydon Indeed.. I can't speak for any other jurisdictions but certainly in the UK there are circumstances where the insurer can refuse to pay out Jul 26, 2019 at 11:38
  • Do "slim jims" work on newer cars ? Jul 27, 2019 at 0:09

I would suspect that this was actually just the main engine cooling fan(s) continuing to run on a recently switched off hot engine.

The fans normally run for up to a couple of minutes after the engine has been switched off until the temperature has been brought down and can be very loud - to the point of sounding like the engine is still running.

  • No, the engines are running ( in TX). Jul 26, 2019 at 15:24

Very surprised to see that it's illegal in Arizona. I wonder if anyone has ever been cited. This time of year (summer) there is usually at least one unattended, running car in front of a big store.

Most cars these days have fobs to lock/unlock the vehicle which makes this foolishness easy. Many new cars (in the US, anyway) have remote start capability, further encouraging this nonsense.

Another subset also use this functionality to allow them to leave their pets/children in the car while they do some shopping.

It's lazy and stupid, in my opinion.

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