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View Full Version : Engine RPMs up or down on shutdown?


srheinz
06-03-2010, 04:18 PM
I've got some conflicting info recently. One dealer told me to always slow the engine all the way down at idle because it's easier on the engine. Another dealer told me to always shut down at full throttle because when you slow the rpms the engine backfires more and that's hard on the head gasket. I'm no expert on small engines but would really like to know what the truth is.

thomasglenn
06-03-2010, 04:48 PM
I've never really thought about it but I always drop the rpms before killing it. I've turned off alot of engines in my time and have never blown a head gasket.

tmanmi
06-03-2010, 05:26 PM
If you have backfire problems you can either let it idle for a minute or so and waste gas or move the throttle just a little off idle and it shouldn't backfire. Never heard of blowing head gaskets with backfire but I did have a grasshopper that blew the muffler out from repeated backfires. Dealer rep told be about shutting down at just off idle, it worked.

mbrew
06-03-2010, 06:06 PM
Mr. James Varnon, a very fine aircraft engine mechanic and instructor taught me over thirty years ago to idle an engine before shutting it down. The reason is as follows: when an engine is at power it is drawing an awful lot of air through the carburetor and fuel along with it. When you suddenly stop the ignition, there is still quite a bit of fuel on the way to the cylinders which will re-liquefy and wash the oil from the cylinder walls and will eventually get past the rings to contaminate the oil. The amounts aren't huge, but over the life of an engine it can become a factor.

I have never run any engine that had a written start up and shut down procedure (this includes aircraft and industrial engines), that they didn't want a minute or so idle before shut down. In addition to the reason above, giving a little bit of a gradual cool down is often given as a reason.

Richard Martin
06-03-2010, 06:32 PM
I always like to let mine idle for a minute or two before shutting them off. If I shut them off without letting them idle first I get a variety of undesireable results. Sometimes the lifters in my Kohler will tick on restart. Sometimes one of my engines will backfire. Other times I'll get a cloud of smoke out of my Kawasakis.

MS_SURVEYOR
06-03-2010, 07:04 PM
I have a 30hp Kohler that has a slight backfire at idle to low rpms. Nothing bad. But when I shut by advice from many, including puppypaws, at 3/4 throttle, I have no backfire. Now my Briggs Vanguard likes to be at idle.

jm2c

ms

Hoy landscaping
06-03-2010, 07:09 PM
mine backfires when its too high and too low. i stick it in the middle and turn the key

turf&tree
06-03-2010, 07:44 PM
MBREW is correct, you are washing the cylinder if you shut down at full throttle.
I get landscapers bringing in equipment all of the time and as soon as they unload it they shut down at full throttle and BANG. I love the business it provides for me. I have another who has all 35 employees mixing 2 stroke at different rates. Wow what they bring me to fix, mostly seized hedge clippers and string trimmers but hey you guys are rich, right?

MS_SURVEYOR
06-03-2010, 07:51 PM
MBREW is correct, you are washing the cylinder if you shut down at full throttle.
I get landscapers bringing in equipment all of the time and as soon as they unload it they shut down at full throttle and BANG. I love the business it provides for me. I have another who has all 35 employees mixing 2 stroke at different rates. Wow what they bring me to fix, mostly seized hedge clippers and string trimmers but hey you guys are rich, right?

Mr. James Varnon, a very fine aircraft engine mechanic and instructor taught me over thirty years ago to idle an engine before shutting it down. The reason is as follows: when an engine is at power it is drawing an awful lot of air through the carburetor and fuel along with it. When you suddenly stop the ignition, there is still quite a bit of fuel on the way to the cylinders which will re-liquefy and wash the oil from the cylinder walls and will eventually get past the rings to contaminate the oil. The amounts aren't huge, but over the life of an engine it can become a factor.

I have never run any engine that had a written start up and shut down procedure (this includes aircraft and industrial engines), that they didn't want a minute or so idle before shut down. In addition to the reason above, giving a little bit of a gradual cool down is often given as a reason.

Thanks! I'll take that under advisement!

ms

hackitdown
06-03-2010, 08:22 PM
Don't the factory manuals say idle?

topsites
06-03-2010, 08:59 PM
READing Teh Pwn'ers Manooal!

Southern Pride
06-03-2010, 10:16 PM
Wow. I can't believe that is even arguable.
ALWAYS start and stop the engine at low rpm.
Upon start up give it a minute or two to warm up. Same for stopping. You want to lower the rpms and let the engine cool down before killing it. Forget the head gasket argument.

Pennington Lawncare
06-03-2010, 10:56 PM
It's better to let your engine idle for as long as you can before turning it off. This is especially true with air cooled engines. Your engine oil is heated to it's maximum temperature when the engine is at full throttle and by idling the engine for a bit before turning it off gives the oil a chance to cool which also helps to prolong your engine oil's effective life.

There are actually some high performance cars that come from the factory with a delay built in so the engine will idle for a minute or so before it dies. This is even more important for turbo equipped engines to let the exhaust temperatures cool down. There are aftermarket kits on the market that can be installed on any car and occasionally diesel trucks to accomplish this.

MileHigh
06-03-2010, 11:00 PM
Shut off at 1/2 - 3/4 throttle if you have a Kohler...

With my kawi's I don't care.

scagmanjosh
06-03-2010, 11:35 PM
Agree with the majority that you shut down after idling a short while.

1.) What other gasoline/diesel/lp/JET A powered machine DOESNT idle the engine before shutting down? Cars? Boats? Trains? Planes? Helicopters? Nope, they all do. Who ever heard of shutting your car off at anything other than an idle?!

2.) Especially important on jet turbine and turbo boosted engines. Allowing them to idle for a minute or so allows the turbines to cool before stopping-- failure to do so can damage seals in them. Oppositely (is that a word?), starting turbine engines too quickly can result in a "hot start", which requires a total teardown of the engine for examination and possible rebuild under FAA commercial aircraft regulations!

My old Kohler Command 16 would detonate/backfire if I shutdown at high throttle. This makes more sense because you are sucking more fuel/air mixture into a hot cylinder, compressing it more-- so even without a spark, it is much more prone to detonate.

I seriously question how shutting down at idle can damage ANY internal combustion engine.....

MOWER ACE
06-04-2010, 09:32 AM
kohler says if you have a fuel solenoid shut your engine off at 1/2 throttle or higher if no solenoid at idle.If more of you guys would learn to read you would probably save some money!!!

K/B
06-04-2010, 09:51 AM
Yep...Kohler had a lot of complaints about backfiring on shutdown on their engines with fuel solenoids. Like was just mentioned, you have to shut the engine off at HALF THROTTLE OR HIGHER.
To quote Kohler:
"Your owner's manual lists the proper shutdown procedures to avoid this occurrence. [backfire] If your engine is equipped with a fuel shut-off solenoid, be certain to set the throttle between half and full throttle before turning the key switch to OFF."

seb442
06-05-2010, 10:48 AM
Wow. I can't believe that is even arguable.
ALWAYS start and stop the engine at low rpm.
Upon start up give it a minute or two to warm up. Same for stopping. You want to lower the rpms and let the engine cool down before killing it.

That's what I do.

My Dad and common sense always told me to start slow to warm up and shut down slow to cool down. I have never done anything different on any engine.

I always start as slow as possible and let engine warm up at least 30 sec to a minute at idle and then gradually advance to full throttle over the next 30 to 45 seconds. On shut down I have always gradually advanced to idle speed and let idle for 30-45 seconds before shut down.

However, the dealer I just purchased a new Grasshopper from (Vanguard 29hp engine) told me to always shut down at full throttle. When I questioned him he said air cooled engines cool best at full throttle and will overheat at slow speeds. Did not mention starting speed.

My Vanguard engine manual says ---
To Start -- Choke and move throttle to fast and start.
To Stop engine -- Move throttle to Slow and turn key off.

I plan to continue my old ways and follow Southern Pride and my Dad.

unit28
06-05-2010, 11:37 AM
I think just because a mower doesn't have a throttle lever like an ATV it's ok
duh!:)

cgaengineer
06-05-2010, 01:10 PM
I shut off at idle, however I do not idle my Kawi for extended periods of time.

MS_SURVEYOR
06-05-2010, 02:36 PM
kohler says if you have a fuel solenoid shut your engine off at 1/2 throttle or higher if no solenoid at idle.If more of you guys would learn to read you would probably save some money!!!

Yea! I went back and reread my Kohler Manual. It states 1/2 throttle or higher. I take heed in the washing of the cylinder when stopped at higher rpms. So I tried idling at low idle before stopping. No backfire! I'll play with that for a while.

Thanks!

ms

rm25x
06-05-2010, 04:30 PM
Couple points...
I do not see any benifit to letting it idle for an extended amount of time before shutting it off. Just bring it down to idle and turn it off. (If your manual says 1/2 throttle, then follow that)

Also, any engine thats at least 10 years old has a fuel shutoff solenoid in the carb. When the key is turned off, the solenoid blocks the fuel from being sucked into the engine. This takes care of any fear of cylinder wash and lessens the chance for backfires.

The reason you get a backfire, is that there is fuel that was not ignited due to the ignition being shut off and then its spent into the hot muffler. Thats where you get your "bang". Its not in the engine, and you can't "blow a head gasket" due to a backfire after turning the key off.

cgaengineer
06-05-2010, 05:32 PM
Rm25x is correct, backfires don't cause blown head gaskets...and any competent mechanic should know that.
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