REFLECTOR: Sniffer valve for Fuel Injected Lycomings (was Idle Adjustment)

[Dave Philipsen]

Keith,

I know a little about this because I'm currently working with an A&P on 
my engine (IO360) and we're installing a sniffle valve.  Most engines 
have an intake manifold which sits below the engine.  It is possible for 
excess fuel to "pool" up in the intake manifold since it's the lowest 
spot.  This could cause a problem especially when trying to start the 
engine.  The excess fuel could cause a backfire or explosion.  The 
sniffle valve works on the principle that there's a pressure difference 
between the manifold and the "outside world".  As long as the engine is 
running, the valve stays closed.  When the engine stops, the pressures 
quickly equalize and the valve opens (by gravity).  When the valve is 
open, any excess fuel that has pooled in that area can drain off.  So, 
the important thing is to locate the sniffle valve at the lowest point 
of the manifold (may even be necessary to use two or more sniffle 
valves) and to mount it vertically so that the valve opens all the way 
when the engine stops.

Apparently, the sniffle valve is not an absolute necessity.  The engine 
in my plane has run without one for some time.  It's simply a safeguard 
against a problem which may occur infrequently.

You can order a sniffle valve from anyone who can get parts for Lycoming 
engines.  I believe it actually has a Lycoming part number.  I've also 
heard of folks who fashion their own.  It cost me about 30 bucks for one 
but, in reality, it might be worth about five bucks.



Keith Hallsten wrote:
>
> Mack, or Sid, or other knowledgeable person:
>
> Please explain this in more detail.  What is the function of the sniffer
> check valve?  
>
> I am not flying (or even running the engine) as yet, but I have the Bendix
> fuel servo mounted on a 100-degree angle adaptor from Airflow Performance,
> which is in turn mounted to the sump of the IO-540-D.  Thus, the airflow
> through the servo will be nearly horizontal.  Exactly what is the 'sniffer
> check valve' and where is it to be installed?  Where would I get one?
>
> Thanks in advance,
>
> Keith Hallsten
> Roseville, CA
>
>   

-- 
Dave Philipsen
Velocity STD-FG
N83DP