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woxof

Pre-takeoff checklist
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woxof
A Cessna 172 that I rent has an issue with the engine. When the mixture is placed at idle cut off, it partially shuts down but still runs on for about 15 second in a rough manner before finally stopping. This happened on three out of four shutdowns today and on the last one, the prop actually started running backward for a few seconds. Lycoming.

Any idea what the problem is?
 
15 seconds? Do you have your fuel pump on? Maybe try to turn the fuel shut off valve or fuel to off next time. The mixture cable could be set up wrong? Just guessing.
 
The behavior is called dieseling, I used to have a very old car that did this when it got hot. Potential causes are many.
One easy one could be the mixture cable has slipped and off isn't fully off.
If injected, there's a fuel servo that could be bad - part cost > AMU
Could be temp related, only does it during the summer.

Some other things to try -
Pull the full cutoff too.
Turn mags off. This still might not stop it.
Squawk the issue and fly a different airplane that isn't broke.

Consider what you would do if it didn't stop and now the engine keeps running with you paying every minute.
 
I had a plane that would sometimes do that. My mechanic couldn't figure out the problem, but shutting the mags off after pulling the mixture helped, and I'm told it can be damaging to a motor for that to continue, so I'd let the owner know what is going on.
 
bflynn said:
The behavior is called dieseling
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Assuming that the O.P. has not shut off the mags, then it's not dieseling like what happens in cars where you shut off the ignition but not the fuel.
bflynn said:
One easy one could be the mixture cable has slipped and off isn't fully off.
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Likely - there has to be fuel for it to keep running.

Shutting of the mags would likely get the engine to shut down cleanly, but wouldn't address the questions of where the fuel is coming from - fuel shut off not shutting off, primer leaking...

But I ain't no A&P.
 
Sounds like a mixture adjustment issue. But you could kill the engine faster if you pulled the mixture and then advanced the throttle. That would lean out the mixture.
 
It sounds like full lean is not all the way to no fuel. It could be a linkage problem or dirt keeping the metering needle from fully shutting off the flow of fuel. Try opening the throttle after going full lean. The added air should force the engine to lean out more and shut down.
 
Metering needle? Fuel pump? Shut mags off? WTF?

Are you pushing carb heat off and leaning for post-landing taxi? Are you sure the primer is closed and locked? For some reason there’s fuel in the induction when you’re shutting down. It could be a pilot thing or it could be a maintenance thing. If carb heat is off and you taxied into parking leaned? Probably a maintenance thing, like the mixture arm isn’t getting full travel. Easy for a mechanic to check.
 
I was thinking something to do with the carb. Isn’t there an idle cutoff mechanism.

I don’t want to open the throttle because it is a floatplane nearing the dock.

Carb heat is on with single mag operation to minimize rpm for approaching the dock.

I was wondering if it started going backwards near the end of the last ‘run on’ due to the of very low rpm after the mixture was pull because the spark is not retarded(I think) causing a kickback to initiate.

It was running backwards for several revolutions, perhaps three or four seconds.
 
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Turn the carb heat off and you'll get a slightly leaner mixture, which may help with shutdown.

And if the engine dies because you pulled the mixture and increased the throttle at the same time, you've solved the problem, not created a new one...
 
woxof said:
I was thinking something to do with the carb. Isn’t there an idle cutoff mechanism.

I don’t want to open the throttle because it is a floatplane nearing the dock.

Carb heat is on with single mag operation to minimize rpm for approaching the dock.

I was wondering if it started going backwards near the end of the last ‘run on’ due to the of very low rpm after the mixture was pull because the spark is not retarded(I think) causing a kickback to initiate.

It was running backwards for several revolutions, perhaps three or four seconds.
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Which motor are we talking about?
 
Carb heat on and single mag is the problem. Both mags, slow idle, lean, and shut it down to coast into the dock. Your notion of low idle to maintain control is providing anything but. You need to reliably shut down before reaching the dock.
 
woxof said:
I was wondering if it started going backwards near the end of the last ‘run on’ due to the of very low rpm after the mixture was pull because the spark is not retarded(I think) causing a kickback to initiate.
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The engine can kick in reverse if the ignition is off and there’s some carbon glow in a cylinder or two. The exhaust loads up with an unburnt mixture under a low RPM and the hot carbon glow ignites and the engine starts turning backward. It’ll continue until it burns it off. The mags aren't involved in the process at all. Like others have said, advance the throttle right after you pull the mix and that’ll choke it out right away.
 
Either the mix control is misrigged or the primer is leaking. The accelerator pump nozzle is ahead of the throttle plate and at idle there is no suction on it.
 
You guys aren’t reading his second post.

I don’t want to open the throttle because it is a floatplane nearing the dock.

Carb heat is on with single mag operation to minimize rpm for approaching the dock.
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He’s slow idling excessively rich on one mag. His “problem” is pilot induced.
 
Stewartb said:
He’s slow idling excessively rich on one mag. His “problem” is pilot induced.
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Which certainly explains it. Excessively rich, low power settings and you’ve got enough lead and carbon left over to start a bullet factory. Get some glow in the cylinders and it’ll surely snap backwards for a few revs.
 
RyanB said:
Which certainly explains it. Excessively rich, low power settings and you’ve got enough lead and carbon left over to start a bullet factory. Get some glow in the cylinders and it’ll surely snap backwards for a few revs.
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Only if you have fuel. Fuel that is supposed to be cut off by the idle fuel cutoff.
"still runs on for about 15 seconds"
That takes fuel.
 
Capt. Geoffrey Thorpe said:
Only if you have fuel. Fuel that is supposed to be cut off by the idle fuel cutoff.
"still runs on for about 15 seconds"
That takes fuel.
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That right there. It cannot run on if there"s no fuel.

I did my floatplane training using the same method. Carb heat and one mag to get the lowest idle possible. Never did it long enough to carbon anything up.
 
Stewartb said:
You guys aren’t reading his second post.



He’s slow idling excessively rich on one mag. His “problem” is pilot induced.
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I doubt it is pilot induced. This is one of several C172 floatplanes used for training. I is standard practice for a one mag-carb heat hot arrival. And it works, repeatedly, over and over all season. So it is likely something else.

Advancing the throttle in such a situation on floats while approaching a dock sounds too risky. I am more interested in identifying the problem at this point for my own knowledge. The company will work on the solution.

Likely a carb issue of some sort. Thanks for the other responses of likely possibilities.
 
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Try it my way and see how it works. Lean it out and you’ll have much better low idle control than running it abnormally rich on a single mag. I’ve got lots of float experience. Your procedure is completely unnecessary.
 
woxof said:
A Cessna 172 that I rent has an issue with the engine. When the mixture is placed at idle cut off, it partially shuts down but still runs on for about 15 second in a rough manner before finally stopping. This happened on three out of four shutdowns today and on the last one, the prop actually started running backward for a few seconds. Lycoming.

Any idea what the problem is?
Click to expand...

Not enough information, there are several different Lycomings powering Cessna 172, could be carburated or fuel injected.
 
My IO-550 had an occasional dieseling issue upon shutdown early this year. It ended up being the mechanical fuel pump. Diaphragm was failing, it also eventually spewed fuel from the drain line attached to it. Mine only stumbled for a few seconds and then shut down. Replaced the fuel pump and 2.5 AMUs later was back in the air. At shutdown, I always do a mag check... mags definitely shutdown.
 
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Where is the mixture set during the one mag, carb heat regime?
 
Hmm, my float training we used idle with no carb heat for water operations. Worked fine. You developed an eye for when to shut down and make the dock.

Hmm, maybe when I didn't have a lot of trouble when I learned to sail. :D
 
To answer some questions above……not fuel injected. Full lean mixture for shutdown. I have done the same procedure on this aircraft previously with no run-pn, so something changed to make it happen several times in a row.

Thanks for the input from somebody that carb heat on is unnecessary. I am not sure how widespread this technique is but I suspect that plenty of pilots use it, including several pilots I flew with.

Here is a manual from Browns in Florida that train many pilots…..

https://www.flymall.org/docs/Seaplane/JBseaplaneManual.pdf
 
kyleb said:
Sounds like a mixture adjustment issue. But you could kill the engine faster if you pulled the mixture and then advanced the throttle. That would lean out the mixture.
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In one of my older clubs this was the policy for shutdowns.. apparently it was also safer as well to prevent an errant kick over
 
I’ll guess the fuel pump reference is in regard to a fuel injected restart version.

There were several early Cessna engine upgrades that required a engine driven pump as well as the electric back-up. The Doyn Cardinal 180 upgrade preceded the factory and used one.

Maybe for when the Law of Gravity is repealed?
 
Magman said:
I’ll guess the fuel pump reference is in regard to a fuel injected restart version.

There were several early Cessna engine upgrades that required a engine driven pump as well as the electric back-up. The Doyn Cardinal 180 upgrade preceded the factory and used one.

Maybe for when the Law of Gravity is repealed?
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cool info! I have zero time in either a restart 172 or an upgraded-needs-fuel-pump 172.
 
woxof said:
I doubt it is pilot induced. This is one of several C172 floatplanes used for training. I is standard practice for a one mag-carb heat hot arrival. And it works, repeatedly, over and over all season. So it is likely something else.

Advancing the throttle in such a situation on floats while approaching a dock sounds too risky. I am more interested in identifying the problem at this point for my own knowledge. The company will work on the solution.

Likely a carb issue of some sort. Thanks for the other responses of likely possibilities.
Click to expand...

This happened to me once in training in a 172M. Pulled the mixture to idle cutoff and it slowed, sputtered, then chugged back to life… repeat for 20 seconds. Stared confused at my instructor and was about to just hit the mags when it quit. I think I remember it kicking backwards too.

The shop said it had a fuel leak somewhere in the carb that was a fairly easy fix.
 
Thanks,

Flew the aircraft again and no issue. Perhaps it was fixed.

In terms of factory C172, the only one I have flown with a boost pump is the 172RG. 180hp carbureted engine. The pump was for emergency use only.
 
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