Why idle before shut off?

[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