Lycoming Hot shutdown "Deiseling" Issue

24Golf · #1 07-10-2017, 07:47 AM

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.

boom3 · #2 07-10-2017, 09:02 AM

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.

airguy · #3 07-10-2017, 09:33 AM

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.

boom3 · #4 07-10-2017, 09:45 AM

I've also tried turning off the mags and it didn't help. I believe since everything is hot, the fuel auto ignites.

Bob Martin · #5 07-10-2017, 10:25 AM

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!

Toobuilder · #6 07-10-2017, 10:32 AM

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.

24Golf · #7 07-10-2017, 12:04 PM

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??

rv7charlie · #8 07-10-2017, 02:09 PM

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

F1Boss · #9 07-10-2017, 02:54 PM

"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

Bavafa · #10 07-10-2017, 03:03 PM

Quote:

Originally Posted by 24Golf

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.