Sniffle valve dribbles

[DanH]

Originally Posted by BillL

So, you dropped it in a glass of water and it did not float? Or is there some more technical test?

I maintain a full time floatation test facility next to the shop. It is not very technical

[rvmills]

Originally Posted by Randy

Thanks for the replies. It is good to know I am not the only one with this going on but I want to fix it. Mine is apparently a little more pronounced than most as I see a good 6" dia. circle on the floor after shut down. I have tried various shut down methods and the situation stays the same. <snip>

Randall in Sedona

Randall, does it look like this? 6" rule in the second pic for ref:




Today's shutdown dribble with yesterday's...I left yesterday's spot just to show you what I see on each shutdown. No oil, just fuel?and it likes to wait till just after I push the plane back into the hangar?just to give me something to clean up! Never thought of it as an engine issue in need of correction though, just normal ops. Wiped up after the photo, so now I have to guess where to push back to next time! FWIW, IO-540 with Bendix servo, 90 deg elbow to an updraft sump. Sniffle valve is on the low point of the elbow.

Not sure what the fox would be short of removing the valve, but its nice that your being considerate of those you visit?last time I went to Rosie's place, I parked on the taxiway so he wouldn't have to chase me with a drip pan!

Originally Posted by DanH

I maintain a full time floatation test facility next to the shop. It is not very technical

Now that's funny raht thar?good thing I don't have one of those?there'd be a lot of twisted metal on the bottom, and fiberglass epic fails floating on top!

Cheers,
Bob

[Randy]

Bob,

Thanks for the photo. Yes, my "spots" look just like that only a bit larger.

For about $300 plus the control cable we can buy the purge valve from AFP and the problem gets solved, with some benefits such as easier hot starts etc.

Randall in Sedona

[flyinga]

Not quite as simple as $300 plus cable. A good bit of plumbing to add too.

[skylor]

Originally Posted by David-aviator

If you had the AFP purge valve there would be no need for the sniffle valve, it does not leak.

Not necessarily. Much of the "fuel drip" after shut down is supposedly because the fuel that's leftover in the injector feed lines boils off and percolates into the intake from residual engine heat. Since the AFP purge valve is actually upstream of these lines, it won't have any effect on this.

Originally Posted by MarkW

I see some pilots push the red knob back in after shutdown. Some say it is safer so they don't hit it while getting in and out of the plane.
The red knob is also known as a ico. It shuts off all fuel going to the servo/spider and therefore the intake tubes. I have had many days after flying that I still have fuel pressure due to this being shut off. This is also a safety issue. If the engine has no fuel there can be no accidental start even if you have a broken mag wire.
If you push the red knob back in after shutdown all the remaining pressure in the system ends up with fuel in the sump therefore draining out your sniffle.

At my local airport, there is a whole group of Bonanza pilots that were taught by one person to push in the throttle and mixture vernier knobs after shut down so that they wouldn't get kicked when climbing out of the left seat. As stated above, this is a safety issue and is really stupid! I know of at least one pilot that knew this and tried (semi successfully) to reeducate these pilots about the dangers and lunacy of this technique!

Skylor
RV-8

[rvbuilder2002]

Excessive leakage from the sniffle valve is a string indicator of a mixture control valve that is not sealing correctly.
The actual valve in a fuel servo is two circular plates with holes in them that rotate against each other. The actual seal is the metal to metal contact of the two plates (no seals). Even at idle cut-off there will be a small amount of leakage while the system is still under pressure after shut down. The normal leakage after shut down is expected to be below what the evaporation rate of teh fuel is, so you should ever see any excessive discharge. Sometimes the seal does not perform as well as expected and the pressure bleeds down at a much higher rate. That will result in liquid fuel flowing out of the sump drain.

[Randy]

I think I had that problem with an injected RV6A that I owned for a short time. In that case, the engine would not shut down cleanly with the mixture control and would stumble and shake etc. I finally adopted turning off the mags to avoid the stumbling.

On my new IO-375, pulling the mixture always stops the engine cleanly, no stumbles etc. The leaking from the sniffle valve will not start for about 5 to 10 minutes after shut down.

I assume from these symptoms that the servo ICO situation is OK.

I do however have a servo that is running too rich at full rich settings, giving me approximately 300F difference in EGT between full rich and peak, they tell me the spec is 200F spread.

Perhaps related?

Randall

[rvbuilder2002]

Originally Posted by Randy

I assume from these symptoms that the servo ICO situation is OK.

Maybe.

A properly functioning mixture valve should be able to hold system pressure for a while (slow bleed down). I believe it is possible to have the valve function well enough to give good engine operation but still leak down too quickly.

One of the airplanes I maintain has had a drippy induction drain since new. The servo was sent out under warranty and it checked out ok but it still has the problem. If the engine is shut down by turning the fuel selector to off it doesn't have a single drip. Any other process (including the high idle for 30 seconds before ICO) and it will drip for hours.

[rvbuilder2002]

Originally Posted by DanH

I maintain a full time floatation test facility next to the shop. It is not very technical

Nice!
And a convenient fire hydrant for keeping it topped off......

[RV7A Flyer]

Originally Posted by flyinga

Not quite as simple as $300 plus cable. A good bit of plumbing to add too.

Or...