Titan-Xpert

Titan-Xpert

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Recently Titan customer support has received some inquires about a blue fuel stain, puddle appearing on the ground under the cowling, about 5 minutes after shut down. The fuel is dripping out of the sniffle valve. This one way check valve is normally installed on most forward facing induction systems, at the bottom rear of the plenum. The sniffle valve closes when the engine is in operation and is open any time it?s not running. Its purpose is to allow fuel from excessive priming to escape the plenum. If a large amount of fuel accumulated in the plenum, it could be inhaled as a large slug of liquid into a cylinder causing a hydraulic lock which can cause permanent damage to a piston or cylinder. If you over prime a fuel injected motor hot or cold it is normal to see this extra fuel leak out of the sniffle valve before starting the engine.
The source of the fuel stain appearing on the ramp 5 minutes after shut down was until recently unknown to us .We asked Mr. Don Rivera of Airflow Performance his opinion of the problem and this is his very knowledgeable reply. Quote

What is happening here is that the engine driven fuel pump is a diaphragm pump. The pump lever compresses a spring in the pump and the spring pushes on the pump diaphragm which sets the pressure and its displacement pushes the fuel out. So at shut down the pump has one stroke of fuel it will displace in the fuel injection system. The fuel leaks out the nozzles then runs down the intake pipes, into the plenum and out the manifold drain. This will happen with all types of fuel injection systems (Airflow, Bendix, Precision Silver Hawk) using a diaphragm engine driven fuel pump unless the system has a purge valve which will dump the fuel pressure at shut down thus eliminating the leakage out the nozzles.
The same thing will happen if the boost pump is energized and the engine is not running. The fuel pressure from the boost pump pushes on the diaphragm in the engine driven pump which compresses the spring in the engine driven pump. Then when the boost pump is turned off the engine driven pump will discharge one stroke of fuel into the system.

Don Rivera
 
Mine has been doing this since day one and the more mysterious thing is that it is not just fuel but some oil with it as I have a large tray in the hanger to keep this area clean. Once all the fuel has evaporated, I see a small puddle (usually a few drop worth) of blue oily stuff in the tray. Of course this keeps accumulating to be a less than a cap full in a month or two.

Lycoming as well as a few mechanic had no logical explanation but assured me is nothing to worry about. It is nearly 500 hours on the engine now and has not stopped nor increased.
 
After landing and while taxing up to the point of shut down, intake manifold pressure is much lower than the pressure in the rocker box, there is about a 10 psi difference at idle. This tends to pull oil from the rocker box area thru the intake valve guide into the intake manifold. After shut down this small accumulation of oil and the fuel that is weeping into the system described by Don Rivera, migrates its way down in the induction system until it finds the sniffle valve were it leaks out. The fuel evaporates and leaves behind a small amount of blue stained oil.
 
I assumed that if the plane were shut down with the ICO and the mixture was left full lean that the servo would not flow fuel even with pressure from the fuel pump on the line. Seams odd some FI planes do this and some don't.
 
On the RSA series injectors, the mixture valve plates can get damaged by foreign materials and will leak at cutoff. It will affect the mixture setting, and folks often adjust the mixture to compensate rather than diagnose and fix the problem. On certified aircraft there is an AD for this issue though.

Airflow Performance injectors are different and can leak past the mixture control at cut off. Not sure as to the acceptability today, but 10 yrs ago they told me that was normal on one I owned. It was interesting that years later I had it overhauled and now it doesn't leak.
 
Some throttle body?s have a small leak down rate, some have more. Maximum leakage of the idle cut off valve inside the throttle body is 5cc/min.
 
Just as a data point I have the Airmotive Silverhawk and I thought mine was a complete shut off without bleed. I checked today and I still has 8# of pressure showing on the system after flying last night. Some 14hours after shutdown.
I think it maybe this that gives me such good hot starts. No fuel has bleed into the cylinder after shut down. My hot starts are usually one blade.
 
The heat that rises off the cylinders, heats the fuel injection lines and Flow divider. As they heat up, the residual fuel in them expands and thus is pushed out the injector nozzle, allowing that fuel to run down the intake pipes to the lowest point in the FI system. After you shut down a hot engine, if you listen after a couple of minutes you will actually be able to hear the fuel expanding and being forced through the injector nozzles. It makes a pretty loud gurgling noise.
Good Luck,
Mahlon
 
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