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.

 

[harpoonalt]

Thanks for the quick replies. He doesn't just not let it idle , he shuts it off at full throttle, then starts it the same way. Drives me nuts....(easily done).

 

[Giles]

Many years ago, I was told by three different mechanics that you should let the engine idle B4 shutdown to let it cool, then race the engine a little and cut off ignition. This helps to prevent starting in basically the same spot on the flywheel.
I don't know if these things are necessary but it is just a habit I have.

 

[RoyJackson]

Many years ago, I was told by three different mechanics that you should let the engine idle B4 shutdown to let it cool, then race the engine a little and cut off ignition. This helps to prevent starting in basically the same spot on the flywheel.
I don't know if these things are necessary but it is just a habit I have.

The cool down period I go along with, but not racing the engine (or, "blipping the throttle" as we call it).
All blipping the throttle does is allow raw gasoline to wash oil off the cylinder walls. That's a habit I lose, if I were you...

 

[Car Doc]

My tractor has a turbo and I let it run @ 1000 for a little while maybe 5-6 minutes etc maybe let the temp gage go down it depends before shutting it down if I have been working it otherwise I just shut it off. :thumbsup:

I don't think it hurts it any but running at a full load and then shutting one down like that other guy does wow I would be ashamed anyone saw me do something like that.

I run my air cooled engines at a fast idle also after working them like my splitter in order to take some of the heat out of it as always ymmv to each his own...

 

[PILOON]

The upper combustion chamber, specially the heads get to be the hottest.
While fluids cool things down the heat is also drawn down the block by simple conduction giving a much larger surface for the coolant to work.
Think heat sink.
Also a running engine circulates the coolant.
Sudden shut down stops all those actions leaving the head as the 'hot spot'.
YES, sudden shut down when at the hottest is detrimental to long life.

In fact due to high temps at the upper ends, some cylinders are actually honed tapered so that when hot the walls remain parallel thereby having the rings always running in a straight cylinder vs a choked cylinder.

Standard procedure on aircraft engins is a short cooling down or idle period to prolong life.

Starting up at high RPM is a big NO-NO as there is no time for the cold thicker oil to get up to the upper areas as well as the bearings.
Also rings rely on splash lubrification which barely occurs with thick cold oil.
Now in the case of hot engine a high RPM start is just as bad because that hot (thin) oil has all dripped down the cylinder walls and you are then running virtually dry rings on dry cylinder walls all while the block is expanded from heat.

It is always best to do a short 'idle' warm up as well as a brief 'idle' shut down.

For shure never lug a cold engine either.

Myself I actually often shift into low gear, low range at idle and 'put-put' along for a few mins as due to the very low gear ratios the tractor is basically only idling anyways even tho it is moving. Also the slow RPM's and slow moving allows all gears ets to get a good coating of lubes before actually getting to work.

 

[Mace Canute]

Many years ago, I was told by three different mechanics that you should let the engine idle B4 shutdown to let it cool, then race the engine a little and cut off ignition. This helps to prevent starting in basically the same spot on the flywheel.
I don't know if these things are necessary but it is just a habit I have.

The cool down period I go along with, but not racing the engine (or, "blipping the throttle" as we call it).
All blipping the throttle does is allow raw gasoline to wash oil off the cylinder walls. That's a habit I lose, if I were you...

I agree Roy. The three mechanics have it wrong too, it doesn't matter what the starting RPM is when they shut it off, the engine will always coast down to the exact same RPM at some point after being shut off. Engines stop at particular spots depending on which cylinder is coming up on it's compression stroke.

 

[Giles]

The cool down period I go along with, but not racing the engine (or, "blipping the throttle" as we call it).
All blipping the throttle does is allow raw gasoline to wash oil off the cylinder walls. That's a habit I lose, if I were you...

There is a difference in raceing the engine a little and "blipping the throttle". Where do you get raw gas by racing the engine a little??

 

[rjkobbeman]

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

Mine does too and I hate it. It is a John-Deere/Scotts/Home-Depot riding mower with a Kohler engine. I thought something was wrong when I brought it home and shut it off the first time. I looked in the manual and it said to have it at half-throttle (or higher) when you shut it off.

Drives me freakin' nuts. So I basically keep it at full throttle all of the time, running or not.

Everything else I own gets idled down first.

 

[ModMech]

Old "wives" tale mostly.....

Tractors and Trucks were the only vehicles with a diesel in this country for a LONG time and they were often lower HP engines than we ahve today. This means that to do the same job they were REALLY working, good for the engine! Some were turbocharged and you should NEVER kill a turbo'd engine after it was working hard or at elevated engine speed because you stop all oil flow when the engine stops and that is generally ALL the cooling the turbo gets! A turbo can spin for up to 90 seconds AFTER the engine stops even at an idle!

It was complete correct to advise people not to shut down an engine that was just working hard, gas or diesel, not a good thing to do. Ditto for one with a turbo, let it idle for at least 15 seconds before you shut it down.

Today because diesels are much more common and used more often in lighter duty applications, it is almost never required that you idle an engine for more than a few seconds before shutting it down. Obviously, if it was in a hard pull immediately before, 2-5 minutes of idle time would be best.

 

[jpm]

well you've gotten good advice so sit back do the right thing and enjoy not having a big repair bill.try not to smile when his motor goes bang