Why idle before shut off?

[harpoonalt]

I was taught (Dad) to always let an engine idle down for a minute or so before I shut it off. My neighbor will be at full throttle, drive over to talk and just shut it off. He starts it back up the same way. Engine racing and off he goes. What's the reason for letting it idle down before shutoff? I thought it had something to do with dumping unburned fuel into the engine but I can't remember. I told him it wasn't good for his motor but he wants to know why and I honestly don't remember.

[stratton]

I know that my lawnmower will backfire every time i just turn it off with out letting it idle down.

[RoyJackson]

An immediate shut down after putting a load on an engine can result in overheating the engine (coolant flow stops upon shutdown and the block becomes a heatsink).
So, a minute or two run at idle (by idle, I mean 1200-1500 RPM with a tractor) allows the coolant to flow at no load and keep the engine cooler.

Worst case if you don't cool it down...warped head.

Now, this applies to a liquid cooled engine. I don't know about an aircooled engine, but I do keep the Snapper lawnmower running a minute or so after mowing. That might just be a waste of fuel.

[TripleR]

I think it helps to let them cool down before shutting them off. Of course on a turbo you have to, but I like to do it on all tractors.

I am not a mechanic, so I may be wrong.

[Hooked_on_HP]

Turbo charged engines need to run without a load for a few min. after having been run under heavy load to let the turbo cool.
Bill

[RoyJackson]

Of course on a turbo you have to...

Good point about the turboes, TripleR

[20 20]

ALL engines should be allowed to have a few minute run down time. I have seen a car that was run real hard then shut off........it seized up the engine solid. It gives the components a chance to come back to temp, before shutting down.

[nikdfish]

More than one reason I suppose.

In the case of turbo charged engines, you want to let them cool down and spin down a bit before shutting off the oil circulation protecting the turbo's bearings. Too hot & the oil can "coke" into chunks that can eventually lead to bearing failure. That's also why I ran synthetic oil in my turbocharged car - to reduce the potential for coking.

I think the other thing is to allow temperatures in the engine as a whole to normalize. When running hard, some areas have higher temps than others, the idle allows the differences to even out before stopping the circulation of coolant.

Just suppositions on my part, someone with real knowledge can probably provide a better description.

Nick


(edit : I took too long typing ... all the bases were already covered!)

[jonyyuma]

More than one reason I suppose.

In the case of turbo charged engines, you want to let them cool down and spin down a bit before shutting off the oil circulation protecting the turbo's bearings. Too hot & the oil can "coke" into chunks that can eventually lead to bearing failure. That's also why I ran synthetic oil in my turbocharged car - to reduce the potential for coking.

I think the other thing is to allow temperatures in the engine as a whole to normalize. When running hard, some areas have higher temps than others, the idle allows the differences to even out before stopping the circulation of coolant.

Just suppositions on my part, someone with real knowledge can probably provide a better description.

Nick


(edit : I took too long typing ... all the bases were already covered!)

Okay, I agree, especially from a turbo diesel engines.Pistons will expand under load and heat. Our gen-sets all have timers, with provisions to cool-down cycle, plus the lubrication is probably pretty thin at that point? We still have to replace some turbos frequently, of course they are on 1000 hp Cummin's engines. Jy

[Ford tractor]

Most of "us" guys that use a tractor for pretty light duty, will have cooled the machine down just getting from where the work was being done to the barn. So, if you were bush hogging, and then shut of the pto and drove to the barn, the tractor will be pretty much cooled off. But if you are bush hogging and your phone vibrates, you should drop the speed to idle and walk off to where it's quiet to talk.

If you don't? there could be some warping issues - but probably not on a non-turbo engine. More likely that the oil film on the cylinder walls will be burned off and greater wear rates will occur on startup.