Why idle before shut off?

[ray66v]

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.

If you have a manual choke, use that to kill the engine.

It won't backfire.

 

[Skyco]

If you have a manual choke, use that to kill the engine.

It won't backfire.

Then you suck alot more unburned fuel in...not a good thing IMO.

As for the guy starting/stopping at full throttle- probably never change his (warped) mind...just remember to never buy anything used from him

My Yamaha Big Bear ATV on shut down makes a nice big thump out the exhaust about 5 seconds later....I've grown a little fond of it:laughing:

 

[brokenot]

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.

A lot of mowers and other smaller equipment is that way. And no, if your manual suggests shutting it down like that, then you shouldn't do otherwise and try to kill it with the choke. Shutting the engine down at half-throttle or so won't hurt a thing in situations like this. What you're doing is basically having the throttle plate open wider when you shut the key off. When you shut the key off, it slams the carburetor solenoid closed. Having the throttle plate opened and fuel solenoid closed leans out the mixture enough to prevent the backfiring, because all the backfiring is, is unburnt fuel in the HOT exhaust system lighting off.

If your fuel solenoid is in working order, then the fuel through the main jet gets shut off immediately no matter where the throttle is. That's why they recommend having the throttle open some. The same amount of fuel will still be able to reach the HOT exhaust, but having the throttle plate open a bit introduces enough air into the mix....and the resulting dilution of the air-fuel ratio keeps the backfires from occurring.

 

[20 20]

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).

That is one piece of used equipment I woouldn't want to own, if it ever came up for sale.

 

[MBDiagMan]

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

Bingo! This is the only need for idling an engine before shutdown. Even in the case of a liquid cooled engine that is very hot. Once it is shut down, where will the heat come from?

 

[TGF]

My instructions said to let it low idle for 5 minutes after working before turning off on hot days, so thats what I do. They also say on cold days let it low idle for 5 minutes before starting work, so thats what I do.

 

[ray66v]

Then you suck alot more unburned fuel in...not a good thing IMO.

IMO it doesn't suck in a lot more fuel, because it quits running pretty quickly.

And unlike turning the ignition off, there is spark longer.

There is already a bunch of unburned fuel in there anyway, it's a carburetor.

 

[Gary Fowler]

I dont know about you guys, but I dont have anything to work my tractor other than turning the wheels anywhere close to my garage, so it take me 5 minutes or more to get to work. If I am using the bush hog or FEL, I will start the tractor then grease up the joints while the tractor is running as I sometimes need to raise or lower the FEl for the joints to take the grease. This usually takes 5-10 minutes. Even if I do no maintenance, I just idle along at 1200 -1500 RPM for several minutes after start up. On shut down, I do the same if going to the garage, I run slow RPM in a higher gear rather than wide open in low all the way then shut the throttle down and turn off the key. I never shut down immediately from full throttle, but I dont have a problem with really lugging my engine on any of the tractors I have as they have ample power to pull anything I put on them without a strain. I do let them idle down for a minute or so anyway to give the turbo a chance to spin down to minimum RPM. It is going to be hot no matter how long it runs even at an idle so you have diminishing returns. Just look at how long the pilots let the jet turbines cool down after they park-not more than a couple minutes and for sure we dont have any diesel engines running as hot as a jet turbine even the turbo hot side. They may use higher grade oil, but it seems the majority of folks here use synthetic blends so we should be ok on that also. Just a note: Synthetic oil should not be used in any motor during break in period.

 

[RoyJackson]

Bingo! This is the only need for idling an engine before shutdown. Even in the case of a liquid cooled engine that is very hot. Once it is shut down, where will the heat come from?

The heat is already there, from use. But it needs to dissipate...which can be by the coolant flow through the radiator or into the engine block/head. One way or the other, it's going to transfer from the coolant. By idling a few minutes, that gives it time to dissipate through the radiator.

 

[Qapla]

To help demonstrate what heat is trapped in the engine, think about this.

When top fuel drag cars finish a run, "smoke" bellows out as they hit the brakes. This "smoke" is NOT the tires.

Many of the top fuel cars have NO radiator. When they reach the end of the run and deploy the brake system, a dump valve opens and dump the water from the engine block into the exhaust system - the "smoke" is actually steam from the hot exhaust vaporising the coolant.

This is done because, if the coolant is not dumped, the residual heat from the engine will overheat the coolant and if the engine stays "closed", it will blow apart.

There may be things they do differently now, I have not kept up with NHRA in recent years. My neighbor is one of the early people of professional drag racing (no, he is not Don Garlits) and he told me the same thing about the coolant and the exhaust as did Kenny Bernstein's team when he was running funny cars.