Why idle before shut off?

[MossRoad]

That's the part that most people don't understand.

I understand that a couple hundred drops of oil is a lot. The oil does not go past the rings. Even if it does, each stroke of combustion burns it off instantly. No worse than "washing" it off with fuel, which I do not think happens in an engine with good rings anyway. As soon as you turn off the key on a car, the spark stops, but the engine keeps spinning for many rotations, which produces suction, which sucks in fuel and air while is spins down. Fuel injected engines probably not so much, but carbureted engines for the last 80 years still sucked fuel and air after the key was turned off. Everything does not stop instantly. There is always going to be fuel and air sucked into a gas engine. I really doubt there is going to be any noticeable wear in an engine if someone gives the throttle a goose before they shut it off. I see no point in doing that, however. Seems strange to rev it up before you shut it down.

 

[Don87]

if oil is on the cylinder walls above the piston rings u should overhaul that motor oil shouldn't be above the rings that is what the oil ring does clean the cylinder walls free of oil or it will some think of that please and the oil ring is the very bottom ring there will be three rings or more on each piston

and nobody pulls into a parking space and holds the throttle to the floor so u really think when u let of the throttle that cools the motor down right then and there wrong look and the temp gauge it doesn't move

think about what ur saying is u can boil water and then turn off the stove and that water will be cooler if thats the case make a vid of u boiling water and then cut it off and stick ur hand in it and lets see what happens
because this is all the same concept here

Wow, I missed this post.
How can I explain this.............rub to pieces of steel together at 4,000 times per minute and tell me what happens with no oil.
Yes, the oil rings on the pistons keep most of the oil in the crankcase, but think about what you said(steel against steel with no oil).
We are looking at 5 minutes tops of run time.
As far as water boiling on a stove, under heat..............that is totally different from an internal combustion engine.
The stove heats from the outside, whereas an engine heats from the inside, which will continue to generate heat, even after the shut down.

 

[whodat526]

We might be misunderstanding each other, or I read the original post wrong.........Run the vehicle for a while, park it, note the temp, shut it down..................turn the key on after 3 to 5 minutes..............note the temp..........there will be a significant increase in temperature.

right we was both reading each others post wrong i understand that after u cut it off it sill go up some not much what is was saying in doesn't cool down by letting it idle for a minute or two before shut down that will not cool it off enough to do anything because like u state the temp will go back up after shut down no a turbo after u let it idle for about five minutes it will cool down some but it would take to long time of idling before the engine cooled any is what is was saying

 

[Don87]

I understand that a couple hundred drops of oil is a lot. The oil does not go past the rings. Even if it does, each stroke of combustion burns it off instantly. No worse than "washing" it off with fuel, which I do not think happens in an engine with good rings anyway. As soon as you turn off the key on a car, the spark stops, but the engine keeps spinning for many rotations, which produces suction, which sucks in fuel and air while is spins down. Fuel injected engines probably not so much, but carbureted engines for the last 80 years still sucked fuel and air after the key was turned off. Everything does not stop instantly. There is always going to be fuel and air sucked into a gas engine. I really doubt there is going to be any noticeable wear in an engine if someone gives the throttle a goose before they shut it off. I see no point in doing that, however. Seems strange to rev it up before you shut it down.

Actually the oil does go past the rings to a degree.


(highlighted)
That's the point.........seems strange............now think, why would it seem strange

 

[ray66v]

where you said if you have a manual choke use that to shut engine down. zman

I offered that to someone as an alternative way to kill the motor, when it has a tendency to backfire.

I assume you realize that if you turn the ignition off, there is still fuel going into the engine which is not being ignited, some of which which would be by the way, if your choking it. How do you deal with the damage from that?

Since your acutely aware of engine damage, what about the damage that backfiring can do to an engine?

My 1980's vintage Briggs engine occasionally backfired, even with idle an idle down, which is why I always shut it down with the choke. It seems to run awfully well, even with all the damage it has suffered as a result of this procedure for all these years. :thumbsup:

 

[whodat526]

Wow, I missed this post.
How can I explain this.............rub to pieces of steel together at 4,000 times per minute and tell me what happens with no oil.
Yes, the oil rings on the pistons keep most of the oil in the crankcase, but think about what you said(steel against steel with no oil).
We are looking at 5 minutes tops of run time.
As far as water boiling on a stove, under heat..............that is totally different from an internal combustion engine.
The stove heats from the outside, whereas an engine heats from the inside, which will continue to generate heat, even after the shut down.

what i was saying is once it hot it will continue to be hot even if u just idle

but tell me why there is no smoke. because i know when i rebuild a engine and lightly coat the cylinder walls with oil it will smoke. the steel rings i believe or many to wear out after so long and so does the cylinder and then needs to be bored out or resleeved. i know i dont wont no oil getting past the rings on any motor i rebuild or i will have some mad customers because there will be some smoke and some blow by in the engine and will be low on compression because if oil and get by the rings so can air is all im saying

if im wrong please tell me i could be very well wrong but i believe im not and if i am i would have learned something new so dont think im being a smart*** i am just curious and dont mind being corrected i like to learn

 

[MossRoad]

Actually the oil does go past the rings to a degree.


(highlighted)
That's the point.........seems strange............now think, why would it seem strange

It seems strange because you are shutting it down so why rev it up.

As for the oil getting past the rings, any that does get past the rings does nothing to lubricate above the rings because it gets burned off. It lubricates below the rings. It gets to the top of the cylinder walls at the top of the piston stroke and is wiped off on the way down.

 

[Don87]

what i was saying is once it hot it will continue to be hot even if u just idle

but tell me why there is no smoke. because i know when i rebuild a engine and lightly coat the cylinder walls with oil it will smoke. the steel rings i believe or many to wear out after so long and so does the cylinder and then needs to be bored out or resleeved. i know i dont wont no oil getting past the rings on any motor i rebuild or i will have some mad customers because there will be some smoke and some blow by in the engine and will be low on compression because if oil and get by the rings so can air is all im saying

if im wrong please tell me i could be very well wrong but i believe im not and if i am i would have learned something new so dont think im being a smart*** i am just curious and dont mind being corrected i like to learn

Yes sir, I understand what you are saying, and I'm sure you understand what I'm saying.

There has to be some oil there, or the rings would heat up and lock the cylinder, but the "light" oil coating also keeps the 'air from entering the cylinder.
After long use(several hundred hours) the rings, and cylinder walls will wear enough to where the engine must be rebuilt to get these 'tiny' clearances back to make the engine work properly.

 

[whodat526]

ok i see your point and can agree on that because the oil will be in the rings groves created during the honing process not the piston rings but in the cylinder its self but it cant be very much oil to change the level like others have stated thats what i was getting at because if thats the cause it would smoke and would be a weak motor

 

[Don87]

what i was saying is once it hot it will continue to be hot even if u just idle

but tell me why there is no smoke. because i know when i rebuild a engine and lightly coat the cylinder walls with oil it will smoke. the steel rings i believe or many to wear out after so long and so does the cylinder and then needs to be bored out or resleeved. i know i dont wont no oil getting past the rings on any motor i rebuild or i will have some mad customers because there will be some smoke and some blow by in the engine and will be low on compression because if oil and get by the rings so can air is all im saying

if im wrong please tell me i could be very well wrong but i believe im not and if i am i would have learned something new so dont think im being a smart*** i am just curious and dont mind being corrected i like to learn

There is always smoke, just because we can't see it with our weak eyes, doesn't mean it isn't there.
You've been around, you know that people have weak eyesight, weak hearing, and weak smell.
Just cause we can't see it doesn't mean it ain't there.