The manual for my mower says I'm supposed to run the engine at low idle for one minute before shutting it down. What's the purpose of this? I'm sure I've never given it a full minute before shutting it down but I lower the throttle while loading it up and strapping it down, probably 20 seconds, tops.

Interesting...the manual for my mower says to turn the key off at full throttle and I've never done otherwise.

I don't know if it's the same reason, but Mercury Optimax outboards are very sensitive to thermal shocking on both start up and shut down. After a long run down the lake, you are supposed to idle for a minute or so before shutting down.

I imagine it is to circulate air over the cooling fins to reduce temp before shutting down.

A

Yep cool down and normalize temps evenly.

My understanding is that is to avoid fuel pooling in the cylinder (which, I have heard, can happen if the engine is revved before shutdown...and to allow it to cool down a little after running hard.

Only my uneducated opinion...

quote:

Originally posted by sigmonkey:
Yep cool down and normalize temps evenly.

Yup.

I concur with comments above...to allow temps to equalize but primarily to prevent excess fuel from collecting in the cylinder and draining past rings into crankcase.

Ever get a little backfire as it's spooling down? The Kohler CH25 on my TurfTiger will do that occasionally if it's really hot and I shut it off without throttling down. Maybe it's somehow related to something like that.

-Rob

quote:

Originally posted by bcereuss:
My understanding is that is to avoid fuel pooling in the cylinder (which, I have heard, can happen if the engine is revved before shutdown...and to allow it to cool down a little after running hard.

Only my uneducated opinion...

Your pretty close.

Revving and engine before shutdown is bad mo-jo, especially for vintage cars (before "good" fuel injection).

The fuel dilutes or washes off the cylinder walls, promotes plug fouling and does no good for the engine.

And similar issues with older carbed cars on start up by "pumping the crap outta loud peddle" on start. Mostly done when a choke is not working properly or by someone who doe s not understand how an engine operates.

So the two practices will result in early ring and top of cylinder wall wear, (because that is the greatest peak of the piston rotational dynamic at the top of the stroke combined with the rocking from combustion and the thrust at a tangent to the rod) high oil consumption, low mileage smoke, herpes, four or five unwanted children and erectile dysfunction.

OK, so it might not cause herpes...

Thanks!

Does it really have to run for a minute or did they just arbitrarily choose that length of time?

Huh, I've never throttled down my Kawasaki on my Toro proline before shutting it off. Hope I haven't shortened its engine after doing this for 15 years now. Guess it's all dependent on the engine and its design.

You have to realize that air cooled engines run around 350 to 400 degrees versus 200 degrees on a water cooled engine.

Mice are the real destroyer of air cooled engines when they build a nest in the engine shroud and block the air flow.

If I have been cutting grass and the engine is at a high temperature, I let it idle for several minutes to lower the temperature before shutting it down.

Gives the exhaust valve time to cool off. To get pollution under control they make engines run hotter. this gets the exhaust valves and guide very hot. If you ever remove the cover and look at the valve springs the exhaust valve will have oil burned and carbon on the guide.On some engines this can cause lack of lubrication and stuck valves

The intake valve is cooled by the intake air and fuel.

quote:

Originally posted by mark123:
Thanks!

Does it really have to run for a minute or did they just arbitrarily choose that length of time?

My guess is that they say a minute in hopes that most operators will give it 15 seconds.

quote:

Originally posted by MNSIG:
My guess is that they say a minute in hopes that most operators will give it 15 seconds.

I try.

You should let it idle a minute if not too..... an air cooled motor actually cools down faster when it's idling versus shutting it off.....it will help the motor last a lot longer as the engine will cool down evenly more running at idle before you shut it off.....

quote:

Originally posted by 12GA:
Interesting...the manual for my mower says to turn the key off at full throttle and I've never done otherwise.

Mine as well.

John Deere D160
Shutting Off Engine
1. Let engine run at high throttle without load for a few seconds.
2. Turn key to STOP position. Engine will stop and headlights will turn off.
3. Remove key.
4. Lock the park brake.

What reasoning do we have for these instructions?

quote:

Originally posted by kimber1911:...
What reasoning do we have for these instructions?

It likely has a shutdown solenoid that cuts fuel flow and that purges any fuel and prevents the issue with unburnt fuel remaining in the intake and cylinders.

And engineers determined that the cooling was sufficient at speeds that a cool down is not required.

Manuals exist for a reason, and if there is conflicting information to the "accepted practice" the conflict is in the understanding of specific operation for that equipment.

My Toro lawnmower is just like my Simplicity snow blower...no throttle, it just runs at one speed. What's up with that?