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?
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.
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.
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.
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.
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.