VAF Forums

VAF Forums (
-   RV Ongoing Maintenance Issues (
-   -   sniffle valve required? (

dromuald 01-11-2020 08:28 AM

sniffle valve required?
I have an RV 8 with an IO-360 injected motor. I bought the plane from the builder 7 years ago. At my recent condition inspection it was suggested that I install a sniffle valve on the bottom of the intake sump. That port is currently capped off with an AN fitting. This motor has 850 trouble free hours on it over the course of 17 years. My question is this - have I been lucky operating this motor or is the valve not really required. I realize it is used to drain the sump of residual fuel and or oil. If there is no drain does this fuel just get scavenged back into the intake system? I guess my point is why should I mess with success?

Bavafa 01-11-2020 09:12 AM

It is not required but it does help very much and it is a great addition for how little they cost or the ease of installation.
It helps drain the excess fuel in the sump when the engine is not running.

bjdecker 01-11-2020 09:14 AM

It would be interesting to measure how much residual fuel, oil, stuff, has accumulated in that sump after all these years...

BillL 01-11-2020 09:28 AM


Originally Posted by bjdecker (Post 1399150)
It would be interesting to measure how much residual fuel, oil, stuff, has accumulated in that sump after all these years...

Yep, next oil change when the engine is hot, take out that plug. If you feel lucky, don't put a pan under it. Mine drips sometimes, especially if flown rich and doing approach practices.

DanH does not have one either . . .

vic syracuse 01-11-2020 09:52 AM

I've done inspections on those without them. Some nasty stuff comes out of that drain when the plug is pulled. It's a combinations of fuel and oil that drains back through the intake tubes when the engine is shut down.


JonJay 01-11-2020 02:21 PM

Probably should note to casual readers that a sniffle valve is for horizontal injection, not vertical.

hgerhardt 01-11-2020 02:59 PM

I haven't had one in 13 years and 810 hours. Last CI I removed the intake tubes to replace the gaskets as I do about every other year (someday I'll install Ross' deal) and as usual there was no more than about a teaspoon of brown motor oil in the back of the plenum. No other "crud".

Also, this being a taildragger, all of that small amount of oil was in the back of the plenum, or several inches behind the sniffle valve boss, so even if there was a valve, nothing would have come out of it.

The only time I could think having a sniffle valve might be useful is IF the airplane were parked in a torrential rainstorm and somehow overwhelmed the drain hole in the fiberglass snorkel. Or if a guy forgot to drill that drain hole... Vic, in your myriad inspections, how many times have you found where there was no drain hole in that snorkel?

Carl Froehlich 01-11-2020 08:54 PM

Install it for a horizontal injected engine. It help prevents blowing out the air filter during a hot start.


DanH 01-12-2020 07:05 AM

Last time I looked in there, it was bone dry. Annual time right now; I'll look again. Not much need to guess, given most of us have some kind of tiny camera on a stick. Open the throttle and poke it in.

As I've written before, there are risks to not using a sniffle valve. The key risk is hydraulic lock...collecting enough liquid in the air plenum (fuel, oil, or water) to enable sucking up an incompressible quantity during cranking, or immediately after start. Risk adverse? Install a sniffle.

Both fuel and water evaporate in due course. They can't build up over time or cycles.

Oil has a very low vapor pressure, thus evaporates sloooooowly, and can potentially accumulate. However, I would think an engine which collects significant oil in the horizontal intake air plenum needs valve guide and/or ring work. Anybody collecting a quantity of oil in their updraft carb airbox? Same mechanics.

Walt 01-12-2020 07:20 AM

The biggest danger is fuel pooling due to over priming/draining after shut down (hot start) and it has no where to go. if the engine backfires into that accumulated fuel it can literally blow off the intake sump or if you're lucky just blow out the air filter if the throttle is open enough to relieve the pressure.

All times are GMT -6. The time now is 12:11 AM.