Idle or turn it off, which is better for an engine?

[BSD]

So I'm sure we've all heard the old "don't shut it down, it takes more fuel to start it again then if you just idled" advice. Based on what I've found (and feel free to correct me) with today's modern fuel-injected engines if you're sitting for longer than 30 seconds it's better to just turn it off in terms of fuel savings.

Now, my question is, what kind of wear does constant on-off-on cycles have on an engine? The closest answer I have ever found is here:

Idling Your Car

However considering that they are environmentalists pretty sure they care more about that .01% less pollution than any (possibly significant) additional wear on an engine. I haven't been able to find out where the got the "only $10 more a year" figure.

Now, my dad has a Suburban (post 2000, forget the exact year) and whenever we went on big trips it was started once and pretty much never turned off until we arrived. If we stopped for food (Edit: Just to clarify, we would get food and keep driving, not sit in a restaurant with it idling) or gas he would let it idle until we left unless we would be there for at least 30 minutes (which pretty much never happened). His argument was that shutting it down would kill the cooling system and allow the engine temp to climb then turning it back on it would be running at a higher than spec temp causing more wear than had you just left it running.

Always made sense to me until I read here about thermosiphoning (http://http://en.wikipedia.org/wiki/Thermosiphon) so perhaps it's better to just turn it off? Consumer Energy Center argues idling causes wear, but how much exactly? It's bad enough that I drive less than a mile to work and during the week my sonic rarely gets up to operating temp if at all...

Also, as a bonus question, my other ride is a KLR650. For the unfamiliar, it's a 1980s designed carbureted, liquid cooled (OOooo High tech!!!1!), 650cc dual sport motorcycle. I remember hearing somewhere that starting is harder on a carbureted motor....No idea if it's true. I've always let it idle, just like my car, unless whatever I'm doing is going to take longer than 10 minutes.

If anybody knows of any studies done or articles, all the better.opcorn:

 

[Stew]

Idling in the 1.4L uses about 0.16 GPH of gas according to my ScanGauge II, a very small amount. I don't think I'd let mine idle for 30 minutes while I ate lunch but I do occasionally let mine idle for 10 to 15 minutes while I return phone calls and such, my car is my mobile office.

Extra wear is just a non issue in my opinion, you will likely get rid of it or the body will rust away before its even starting to be a concern.

 

[ThatTmoGuy]

If I'm sitting for more than a few minutes ill shut it off, also if I'm sitting for a minute in the drive through to make it easier on others ordering.

Even on older vehicle's this is still true, its not like its going to take up that much extra gas to start it. That's just some crap some idiot came up with to sell you more gas.

 

[Eric Smit]

I generally only shut mine off when I'm going through the drive through or I have arrived at my destination. I've never left it idling for lunch though, that's for sure.

 

[chivalryder]

... shutting [the engine] down would kill the cooling system and allow the engine temp to climb then turning it back on it would be running at a higher than spec temp causing more wear than had you just left it running.

I'm curious... When you shut down an engine, there is no longer any combustion and the parts stop moving, so there is nothing happening in the engine that produces heat. What would cause the engine to continue to increase in temperature that the cooling system would have to cool down?

I do believe his statement that the cooling system shuts down completely when you kill the engine is false. In my parents '04 Cavalier, I had driven around an indoor parking garage during a heat wave of summer and the engine temps were climbing quickly while cruising around in 1st. It forced the electric fan to turn on (that thing is LOUD!), and it stayed on for a good 3-5 minutes after I turned the car off and even left the car. There's no way a (modern) engine can overheat after you turn it off.

The #1 thing I would be worried about with constant starting/shutting off the engine would be the oil throughout an engine dripping out while the engine is hot and turned off, then the lack of oil in the engine upon restarting. While leaving it at idle, at least the engine will have a steady flow of oil throughout all the internals.

However, modern engines are likely designed for hot starts, so in a new engine, this would not be much of a concern.

Also, most municipalities are enforcing a 3-minute idle law. You might as well just shut the engine down.

 

[Patgt500]

The term you're looking for is Heat Soak, when the engine is no longer being actively cooled internally. The cooling fan can continue to run after the engine is shut off, but that doesn't do alot for the coolant in the coolant jackets.

 

[Maddog2020]

Seems like a bad idea to pump gas while the engine is running.

 

[BSD]

The term you're looking for is Heat Soak, when the engine is no longer being actively cooled internally. The cooling fan can continue to run after the engine is shut off, but that doesn't do alot for the coolant in the coolant jackets.

See, this is what I always thought. So the question then becomes which causes more wear: idling or heat soak? From a quick search, heat soak only lasts 3-10 minutes before the engine starts to cool. By that number, you shouldn't idle more than 10 minutes and you should wait 10 minutes before restarting to minimize wear.

Seems like a bad idea to pump gas while the engine is running.

I've done it in every vehicle I own (sans motorcycle), my dad has done it for 30+ years. Both of us are still here. I'll quote from this thread:

Idling Car while Filling up w/ Gas - Straight Dope Message Board

This was the subject of a lengthy thread last June. I'll say now what I wrote then... I don't think there's any evidence that leaving the engine running increases the risk of fire or explosion. In that thread, nobody could produce a cite showing a single case where a gas-station fire was caused by leaving an engine running, and I don't believe anyone offered a rationale explanation for why it's dangerous to leave an engine running while the gas is being pumped, but not dangerous for the engine to be running while the car is entering/exiting the pump area.

I have no problem with avoiding the practice on "it couldn't hurt" grounds, but I haven't yet heard an explanation for how a non-defective running engine could spark an explosion while pumping, but not pose the same risk during normal operations.

If anyone can cite an actual occurrence, or can explain the cause of the elevated risk, I'd be happy to stand corrected.

 

[Swordsmith]

I shut the engine off if I get out of the car, and if I am sitting waiting for someone who might be more than a minute or so. No, I have not done the math to see whether one way or the other uses more fuel, although it's hard for me to imagine that a 5 second start operation uses more fuel than 5 minute of idling, either way it's miniscule. This is just the way I learned to do things.

Getting out of the car means I've abandoned a potentially lethal object with no operator, in case it mysteriously slips into gear or something. Paranoia perhaps, but man that idea just gives me the shakes. Funny since most scenarios I imagine for this involve automatics and I always drive manuals...

 

[1980PontiacWagon]

Not sure how much heat soak happens in the sonic, but I do know that the turbo gets HOT, along with the exhaust manifold, which are under the hood. Sure the turbo/manifold has a "heat shield" but its also right behind the radiator.

"Heat soak only lasts 3-10 minutes" I highly disagree.

I don't have real numbers for the sonic because it doesn't have a temp gauge. BUT in my wagon with a mechanical temp gauge I know that on a summer day after driving for a while my temp stays steady at 180*F, but when I shut it off it shoots up to 220-240*F. and it will stay there for 30-45 minutes.

And I think engine wear has more to do with, how a person drives, the maintenance done on it, and the quality of the fluids put into you engine.