Home > VansAirForceForums

-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

Old 01-11-2020, 08:28 AM
dromuald's Avatar
dromuald dromuald is offline
Join Date: Aug 2012
Location: Newnan, GA
Posts: 48
Default 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?
Dave Romuald (FFC)
RV-8 655CL
RV-14 in work
Reply With Quote
Old 01-11-2020, 09:12 AM
Bavafa Bavafa is online now
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,955

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.
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - SOLD
N258SM RV14A - IO390EXP119 - Out of Paint shop & flying
Dues paid
Reply With Quote
Old 01-11-2020, 09:14 AM
bjdecker's Avatar
bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 1,248

It would be interesting to measure how much residual fuel, oil, stuff, has accumulated in that sump after all these years...
Brian Decker
RV-7 (Flying)
Reply With Quote
Old 01-11-2020, 09:28 AM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,682

Originally Posted by bjdecker View Post
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 . . .
Reply With Quote
Old 01-11-2020, 09:52 AM
vic syracuse vic syracuse is offline
Super Moderator
Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,729

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.

Vic Syracuse

Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES/RW
Kitplanes "Unairworthy" "Diagnostics" monthly feature
EAA Sport Aviation "Checkpoints" monthly column
Author "Pre-Buy Guide for Amateur-Built Aircraft" & "Are your Nuts Tight?"
Reply With Quote
Old 01-11-2020, 02:21 PM
JonJay's Avatar
JonJay JonJay is offline
Join Date: Jan 2007
Location: Battleground
Posts: 4,342

Probably should note to casual readers that a sniffle valve is for horizontal injection, not vertical.
Smart People do Stupid things all the time. I know, I've seen me do'em.

RV6 - Builder/Flying
Bucker Jungmann
Fiat G.46 -(restoration in progress, if I have enough life left in me)
RV1 - Proud Pilot.
Reply With Quote
Old 01-11-2020, 02:59 PM
hgerhardt's Avatar
hgerhardt hgerhardt is offline
Join Date: Oct 2006
Location: torrance, ca
Posts: 810

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?

Last edited by hgerhardt : 01-11-2020 at 03:02 PM.
Reply With Quote
Old 01-11-2020, 08:54 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,752

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

Reply With Quote
Old 01-12-2020, 07:05 AM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,862

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.
Dan Horton
Barrett IO-390

Last edited by DanH : 01-12-2020 at 07:08 AM.
Reply With Quote
Old 01-12-2020, 07:20 AM
Walt's Avatar
Walt Walt is offline
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,727

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.
Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
Website:, Email:, Cell: 972-746-5154
Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -6. The time now is 08:39 PM.

The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.