What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Lycoming Hot shutdown "Deiseling" Issue

24Golf

Well Known Member
I have noticed this problem occasionally over the last few summers, but this year it has gotten really bad. After flying any amount of time (1 to 3 hrs or more), when I land and roll out to the fuel island to fill up, I pull the mixture to cutoff and the engine ALWAYS dies quickly (like it should). BUT, when I crank up and idle down to my hangar and kill the engine by pulling the mixture, I get a horrible "dieseling" that lasts for several seconds and is somewhat violent. This problem is obviously heat related, since it only does this in the summer. I have tried several things, none have helped:
Running the engine at 1500 RPM for several minutes to try to bring in cooler fuel.
Pulling the mixture to near cutoff for several minutes at low and high RPMs before cutoff.
Cutting off the fuel flow at the valve (made it worse).

Any ideas?? The engine is a factory Lycoming with just shy of 900 hours on it. The injection system is the Precision Airmotive EX-5VA1 and I am running 2 P-mags with all auto plugs and 100LL only.
 
I usually only experience this at Oshkosh when I have to taxi for a half hour in the afternoon heat before shutdown. I've tried turning off the fuel entirely and it doesn't help. I'm not sure if it would help anyway, but at Oshkosh it's tough to do a good run up before shutdown with all tents around.
 
What you're experiencing is high temperature fuel in the servo and injector body boiling off and continuing to supply a mix of liquid and vapor to the injector servo after you've cut off the mixture to the servo. Basically all the fuel in the servo itself and the line going from the servo to the injector divider, plus the fuel in the injector divider itself, is flashing into vapor and continuing to feed the engine.

You can avoid that with two methods - kill the engine with the ignition under these conditions, in which case it will stop running immediately - or use the Airflow Performance purge valve to provide a hard cutoff of fuel flow at the injector divider. I have the purge valve on my airplane, and it gives a hard and immediate shutdown under this scenario. More importantly it allows me to recirculate cool fuel from the fuel tank through the servo and up to the injector, then back to the tank to cool off those components and fill them with cool fuel before a hot-start after heat-soaking on the ground. At that point the engine acts just like any other cool start attempt without loping along in a rough idle for a while.
 
Last edited:
I've also tried turning off the mags and it didn't help. I believe since everything is hot, the fuel auto ignites.
 
I was told by a friend the Comanche's used a higher than normal compression ratio and their throttles had a normal idle dedent but also had a way to move the throttle and completely close the throttle body to stop air from flowing. For this very situation. Thought was high temps and high CR mixed with carbon build up on the piston/combustion chamber created a perfect storm for the dieseling/running on....and completely choking off the air supply was the fix. Just another thought!
 
Closing the throttle blade should do it. Back in the days when cars had carbs, they used a solenoid to close the throttle below the normal idle position when the ignition switch was turned off.
 
Is there a safety issue with killing the mags before the last turn of the prop?? What about killing the mags and pulling the mixture at about the same time??
 
Have you tried to *open* the throttle (make the mixture too lean to burn)?

Flame suit on, but it's kinda the inverse of the hot start technique many use, where you begin with the throttle wide open to get the mixture lean enough to light.

Since it's a new symptom, I'd bet on the spider as the cause...

Charlie
 
Plan B

"BUT, when I crank up and idle down to my hangar and kill the engine by pulling the mixture, I get a horrible "dieseling" that lasts for several seconds and is somewhat violent."

Well, the AFP shut off valve will certainly get rid of the problem. My guess is you want a right-now fix!

I had a 540 doing this to me, and Don's advice was to run up to 1200 then pull the mixture to ICO while opening the throttle to 1/2 or more. This was before the AFP recirc valve was made up (Ancient times, ya know). Worked every time.

Do this away from your hangar a few times to make sure it works in your case.

Best,
Mark
 
Is there a safety issue with killing the mags before the last turn of the prop?? What about killing the mags and pulling the mixture at about the same time??
Not on technical side. The primary reason for pulling the mixture for killing the engine is to ensure there is no fuel in the system in case some one moved/turned the prop and the engine fired up. so some caution will be required.
 
Have you tried to *open* the throttle (make the mixture too lean to burn)?

Charlie

I had a 540 doing this to me, and Don's advice was to run up to 1200 then pull the mixture to ICO while opening the throttle to 1/2 or more. This was before the AFP recirc valve was made up (Ancient times, ya know). Worked every time.

Do this away from your hangar a few times to make sure it works in your case.

Best,
Mark

These were my thoughts too. The fuel is getting there anyway, so just flood it with air. A few extra rpm allows the spin down to exceed the fuel flow.

It may be a little more rough on the last revs as compression reaction torque is higher with the throttle open.
 
Is there a safety issue with killing the mags before the last turn of the prop??
It's how every car on the road turns off, and they do it without a "mag check". The safety issue is ensuring the mags are indeed "off" when "off". A mag check can confirm the switches work (or did, the last time they were moved). The benefit to pulling mixture is that there's no fuel left in the carb, or in your case in the injectors, which is a "backup" safety step. If the mags are off, any fuel left in the injectors won't ignite anyway.

When I pull up to the fuel pumps, I *always* shut down with the mags. No sense in burning the fuel out when I just have to fill it up again 5 minutes later. Once fuelled, switches on, starter, and it picks up where it left off. Parking for the night, I do still pull the mixture.
 
We have a Cessna 150 in the club that will do the same thing, but every time you shut down.

It has a continental 0200. Throttle up to 1200 and pull the mixture to cut-off. Smooth shutdown every time. That is also written in the engine manual.
 
For those of you who are carbureted, I learned something last week: idle cutoff does not cut off the fuel, it reduces the fuel flow to the point that combustion stops at that particular rpm.
While troubleshooting an engine problem, I wanted to get compression readings using an automotive tester instead of the differential tester.
So, with all top plugs out and the tester screwed into one cylinder, with the engine cold and the mixture at cut off, I stood outside where I could see the gauge while reaching in and activating the starter.
Much to my surprise, the engine immediately fired up and happily chugged away on one cylinder until I turned off the ignition, at which point all the loose stuff in the hangar stopped swirling around and slowly descended back down.
So; don't do what I did!
I seem to be saying that a lot, lately...
 
Much to my surprise, the engine immediately fired up and happily chugged away on one cylinder until I turned off the ignition, at which point all the loose stuff in the hangar stopped swirling around and slowly descended back down.
Hmm... Why were the mags on? :)
 
AFP FM-150 on a IO-320 with two PMAG's ...

Hello

FM-150 on a IO-320 with two PMAG's ...

I switch off like this all the time:

- Set 1000 RPM (at least)
- Pull mixture 2/3 to 3/4 of its way
- After about 4s, pull mixture to idle/cut off
- 2s later turn OFF Ignition

So far, I do not have "dieseling" of engine.
Without turning the ignition OFF, yes I have "dieseling" ...
 
Hello

FM-150 on a IO-320 with two PMAG's ...

I switch off like this all the time:

- Set 1000 RPM (at least)
- Pull mixture 2/3 to 3/4 of its way
- After about 4s, pull mixture to idle/cut off
- 2s later turn OFF Ignition

So far, I do not have "dieseling" of engine.
Without turning the ignition OFF, yes I have "dieseling" ...


THANKS GUYS! I have a lot of things to try now... Dominik, I like your method the most, will try that first...
 
The engine ran because I had left the bottom plugs in and connected. I removed all the top plugs and grounded those leads. So it ran on one cylinder, the one that had compression from having the tester in the top plug hole.
The point that I was trying to make is that the engine may start when cold, even with the mixture at ICO...
 
Dieseling Lycoming Airflow Performance w/purge dual pmags

I'm getting dieseling on my io-360 Lycoming with an Airflow Performance FM-200 w/purge valve and dual pmags.

It has happened the last two times I flew. The only material change is that I reduced the main jet size from 49 to 49.5 in my FM-200 because my full rich fuel flows were too high.

I am using UL91 https://www.aviation.total.com/fuels/avgas-ul-91 fuel, not 100LL. Just filled up - and I wonder if I got a bad batch?

The dieseling happened even by cutting the purge valve, which means that no fuel should go to the spider, and even cutting the pmags. The second time I had the engine running at 1200 RPM before cutting mixture, purge, and then pmags.

I looked at the pistons with my borescope, and they look perfectly clean, with the only "sharp" things on the piston are where the pistons were stamped with '1', '2', '3', '4' - otherwise very smooth. Valves looked perfect - which is normal since I have about 20 hours on the engine.

I'll try running 100LL to see if it changes anything - was hoping to avoid running lead, but I'm out of ideas. Thanks for any hints.
 
I'm getting dieseling on my io-360 Lycoming with an Airflow Performance FM-200 w/purge valve and dual pmags.

It has happened the last two times I flew. The only material change is that I reduced the main jet size from 49 to 49.5 in my FM-200 because my full rich fuel flows were too high.

I am using UL91 https://www.aviation.total.com/fuels/avgas-ul-91 fuel, not 100LL. Just filled up - and I wonder if I got a bad batch?

The dieseling happened even by cutting the purge valve, which means that no fuel should go to the spider, and even cutting the pmags. The second time I had the engine running at 1200 RPM before cutting mixture, purge, and then pmags.

I looked at the pistons with my borescope, and they look perfectly clean, with the only "sharp" things on the piston are where the pistons were stamped with '1', '2', '3', '4' - otherwise very smooth. Valves looked perfect - which is normal since I have about 20 hours on the engine.

I'll try running 100LL to see if it changes anything - was hoping to avoid running lead, but I'm out of ideas. Thanks for any hints.

I am surprised you are getting dieseling if you are using the purge valve?
Are you using the purge valve for shut down and leaving the mixture control in?

I am planning on installing a purge valve on my upcoming build. Have you talked with Don at AFP about this?

Please report back and let us know how you solved this.

Best,
Jim
 
The dieseling happened even by cutting the purge valve, which means that no fuel should go to the spider, and even cutting the pmags. The second time I had the engine running at 1200 RPM before cutting mixture, purge, and then pmags.
.

Auto fuel should not self-combust at 9:1 compression ratio, regardless of the AFR. Therefore, if you get "run on" after the ignition is turned off, you have a "hot spot" (usually a small chunk of carbon) somewhere in the cylinder that is acting as a glow plug, as well as enough fuel mixed in with the air to support combustion. This assumes the ignition is truly off, which can be confusing with the Pmag. Rough running with just ICO (Ign still on) is somewhat common due to the vaporization issues outlined earlier.

Larry
 
Last edited:
Here is a link to AFP website and specifically how the purge valve option works. In addition, it talks about how the diaphragm fuel pump acts like an accumulator and hold pressure when the engine is shut down.

That should all go away and fuel and pressure should be eliminated if the purge valve is placed to idle cut off upon shut down and the mixture knob is not used in the process.

Not sure why the engine would continue to run when using the purge valve as referenced in post 22?

Link:
https://airflowperformance.com/wp-content/uploads/2016/11/Purge-Valve-Manual-8-28-07.pdf
 
i agree

...

Not sure why the engine would continue to run when using the purge valve as referenced in post 22?
...
I agree, which is why I'm seeking info from the brain trust.

I've attached what I think is the strangest thing I saw on my pistons. Looks like someone stamped one number, then re-stamped another. Or something terrible is happening! :eek:

Screen Shot 2020-09-12 at 13.02.36 .png

The others look like this:

Screen Shot 2020-09-12 at 13.04.16 .png
 
Dieseling Lycoming Airflow Performance w/purge dual pmags

I'm still getting this issue - cutting with mixture, fuel purge, even pmags. I even tried one tank with 100LL - same problem, although possibly a bit less - OAT is also changing, so hard to constrain all the variables. I must have a "hot spot" somewhere. I will try using the fuel purge with the RPM higher. Normally it's about 900-1000 when I shut down. I might also try replacing the iridium plugs with normal plugs - perhaps there is a "hot spot" on the fine tip.
 
Back
Top