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Sniffle Valve Drain Down

Champ

Well Known Member
I have always gotten a little leakage from my sniffle valve on shutdown so I put a syringe on the outlet port to keep from staining the hanger floor. Gas with a tinge of oil to blacken it. I shutdown with ICO.

Lately the leak down has gotten quite a bit higher - see the picture.
Sniffle Valve Drain.jpg

Any ideas as to why so much?
 
when i shut down via ICO and leave the mixture alone, only a few drops come out of the sniffle valve. but when i push the mixture forward after shutdown it's a similar amount of gas/oil as in your picture.

that leads me to believe that your ICO in the throttle body is not to 100% leak free.
 
IMO it has not to do with ICO .. it is the amount that is in the divider, that fuel up there will start to "boil" after shut down. Then get pushed into the injector lines, where they drop into the cylinder and accumulate on the lowest point. Via the sniffle valve, you get rid of it.
 
Pilot controlled . . .

It got cooler outside and you are running way rich in the pattern and taxi.

I get this when doing approaches and if I pay attention to leaning, the drip is non existent, otherwise - lots.

Try it and track the changes. :D

The servo and mix valve does have a controlled leak, to say it is seldom perfect, very small and can be related to pressure. I discussed this with the manufacturer when we were having the high pressure failures of the fuel pressure sensor. There are a couple (at least) different types. Probably not a failure of this part, but operation technique. A friend kept a bucket under his, and it had a gallon of goo in it after years.
 
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Good points.

OAT has been pretty high on the last flights and flashing in injector lines is noticeable by the after shutdown sizzle. That fuel vapour could get thru an open intake valve and then condense on its way to the intake manifold and come out the sniffle valve.

I always lean after start up and while taxing to keep from fouling plugs but have not been leaning after roll out and while taxing back to the hanger. It might help to lean then and even run lean for a few minutes at 1200 rpm+- before shutdown.

Thanks. I’ll keep these points in mind on my next flight.
 
Dennis - which FI system do you have? I have the AFP FM200 with a purge valve, and the ICO does not cut off fuel completely, which is why they recommended the purge valve. When I am at idle, and I pull the mixture back fully to ICO, the engine will keep running - just barely. This is documented behavior for this model of FI. The purge valve stops the engine immediately.
 
That fuel vapour could get thru an open intake valve and then condense on its way to the intake manifold and come out the sniffle valve.

The fuel nozzle is located directly above the intake tube junction on the head, about where I've placed the red dot below. Post shutdown, dripping fuel can't enter a cylinder. It goes down the intake pipe to the sump plenum, and given enough volume, out the sniffle valve.

Just polishing the pins...
.
 

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My FI is a Precision Aeromotive Silver Hawk EX-5VA1. No purge valve. ICO does shutdown the engine completely after a leaning rise in rpm.

Dan - I see the injector is upstream of the valve seat - makes sense. I thought it was in the head above the valves. Much better atomization of fuel coming into the cylinder where it is and cooling the valve & seat. There is a good path for injector leakage after shutdown to end up in the intake manifold.
 
Your signature line says IO375, so I'm assuming you have a Superior "cold air" manifold. It has a small plenum. The photo below is a standard Lycoming horizontal sump, which has a large plenum. It was taken with a borescope inserted through the throttle body, so the intake tubes are #3 on the left and #4 on the right.

The Lyc horizontal sump incorporates tuned length intake tubes, and as you can see, they sit well above the plenum floor. The tapped hole is the sniffle port. You can see a little bit of fluid there, and a little more in the back of the plenum. This is a tailwheel aircraft, so anything dripping from an intake tube runs to the back.

Point here is that the small plenum Superior sump may vent almost every drop to the sniffle, while a Lyc sump on a tailwheel aircraft can store some volume in its large plenum before much shows at the sniffle. A slight change in leak rate may be very evident with a small plenum manifold. An aircraft equipped with the Lyc sump may be leaking a lot more fuel after shutdown, and not show it.

Photo on the right is looking into the sump plenum with the pipes removed.
 

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Sniffle Valve Status

Dennis, do you ever find out what was causing your sniffle valve issue? I have the same problem. Any suggestions?

Bob Cowan
RV7A
 
It might vary with how I lean before shutdown. It’s generally less than the picture I posted. I still use a modified syringe to catch it and keep from staining the hanger floor and don’t worry about it.
 
Catch pan $0

I use an old scratched up Teflon fry pan with a handle that was destined for the junk yard. Made perfect sense to me. Another RVr came by and said, “Wow, that’s a good idea. Never thought of that.”
It’s also where I throw/drop all my safety wire bits when removing them from the engine during oil change and other maintenance. EZ to pick up and clean afterwards.
 
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