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Old 04-28-2020, 06:54 PM
laserman laserman is offline
Join Date: Nov 2013
Location: Auburn, NY
Posts: 70
Default Leaving the oil filler cap ?Open? to vent moisture ? (For a bit)

So I can’t find any thoughts on an open oil filler cap to let engine moisture escape from a hot engine ??

So after a nice one hour plus flight today , after a fresh oil change, on a 300 hour well maintained engine (oil temp running at 185F). I pulled the aircraft is back in hanger .. I pulled the dip stick and again saw the milky white oily mixture on the filler cap / filler neck (clearly water in the oil) and watch the vapors pour out..

I’m sure it was a mixture of mostly water and maybe some hydrocarbons.. I then watched as the water continued to condense on a good sized and cold (35F) chunk of aluminum I held over the open oil filler neck. I mean a ¼- ½ teaspoon over a 10-15 minute period. (yes engine breather tube clear)

With the oil filler cap left on.. There is no way that ½ teaspoon of water is ever going to get out of the engine .. A 2’ long 3/8ID crankcase vent tube.. that’s open ONLY BELOW the engine.. sure is not going to let the moisture escape..

It really made it clear why these aircraft engines rust for the inside out.. A sealed up engine has no chance for a reasonable life unless its flown A LOT.

So can it be as simple as leaving the oil filler cap off for a few hours... to a day.. Letting the moisture “Vent out of the TOP of the engine... ? Enhancing the moisture dissipation by leaving the engine heater on for a few hours ( Cylinder & oil pan heaters) Yes.. There is no way you would want to leave an engine heater on with a “Sealed-Up” engine.. The moisture just able to circulate inside the crank case. Rust.. Rust ..Rust...

So there is a nice $49 monitor module (amazon) that will record temp/humidly every minute for up to 20 days and then be downloadable to a smart phone or Excel .. With a slight mod.. and remote wiring of the sensor, so I can have the sensor in the crankcase after a flight.. I’m going to get some real data on this idea ..

Anyone else looked into the simple moisture venting concept?

Experiment #1

Nice flight.. Let cool over night ..

Next day.. Measure internal engine humidity for several days

Experiment #2

Nice flight.. Leave oil cap off.. With engine heat for 6 Hours.. Heat off.. Cap off..

Next day(1) .. Replace oil cap.

Next day(2) ..Measure internal engine humidity for several days

Give me a 2 weeks and I will have some real data

Al Karpinski
2020 - RV8 N498RV ..( aka “LipStick”) - Phase 1 Completed 3-21-21..
Now at 50+ hours of RV8 Fun
2016 182T N522CS
Home Airport B16.
EAA 486 @ KFZY...
2021 Dues Paid.

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Last edited by laserman : 05-13-2020 at 04:20 PM.
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Old 04-28-2020, 07:02 PM
jrtens's Avatar
jrtens jrtens is offline
Join Date: Apr 2018
Location: Utah
Posts: 218

Venting after flight SOP for me.
2019 12iS SLSA
Salt Lake Int'l
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Old 04-28-2020, 07:12 PM
pa38112 pa38112 is offline
Join Date: Mar 2008
Location: Clarksboro, NJ
Posts: 1,004

The problem I have always had with this is that I cant stand around long enough to fully vent it, and if I leave the dip-stick open I forget to come back later and close it...
2004 RV6A Flying
2002 RV6A Flying
1978 PA38-112
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Old 04-28-2020, 07:18 PM
plehrke's Avatar
plehrke plehrke is offline
Join Date: Sep 2006
Location: Defiance, MO
Posts: 1,828

I vent after ever flight. Probably get most of the steam to escape in first 10-15 minutes while I am wiping down the plane and filling out logbook. Put stick back in before I leave as don’t want critters to crawl down the dip stick.
RV-6A - flying 970+ hours
Based at 1H0 (Creve Coeur)
Paid dues yearly since 2007

Last edited by plehrke : 04-28-2020 at 07:23 PM.
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Old 04-28-2020, 07:25 PM
Scott Hersha Scott Hersha is offline
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,018

If you fly frequently, like 4 times a week, you probably won?t have to worry about it. But - I?ve never considered this ?venting? procedure and will be checking it out. In the colder months when I can?t fly as often, it seems like a sensible thing to do....
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
RV6/Used kit purchased 2021 building
Cincinnati, OH/KHAO
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Old 04-28-2020, 07:48 PM
tgmillso tgmillso is offline
Join Date: Nov 2012
Location: Launceston, Tasmania, Australia
Posts: 841

I fully agree with the above post. There's just too much risk in forgetting. With my dehumidifier I never remove the dipstick unless I'm immediately ready to install the dehumidifier bung and vice versa.
Regarding the humidity measurements, the way I have my unit set up is that the air exiting the crankcase goes into a jar which has a household, AAA battery powered humidity sensor. After shutdown my humidity sensor typically reads 70-80%, then drops down to 10-15% after a few hours of pumping dry air in from the dessicant.
Once things get to about 30-35% (takes about 40 engine starts to achieve this) I put the dessicant in a baking tray in the oven for about 105?C and it's good as new.
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Old 04-28-2020, 08:01 PM
thinkn9a thinkn9a is offline
Join Date: Nov 2007
Location: Mississippi
Posts: 309
Default Experiment 3 ??

Forced air vent through breather tube during initial cooling phase.. with fish tank pump, or ballon inflator or other simple pump
Wallace & Marietta Goodloe
9A -QB
Phase 2 has started!
Thanksgiving time, is dues time for us
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Old 04-28-2020, 08:21 PM
Jetmart's Avatar
Jetmart Jetmart is offline
Join Date: Sep 2019
Location: Windsor, Ontario
Posts: 435

SOP for me and would leave oil sump and cylinder heat on continuously.
No chance of forgetting the cap off if you check your oil before every flight.
Glenn Martin
Windsor, Ontario
1942 Tiger Moth
2017 Waco YMF-5
RV-14 Kit # 140694
First Flight - June 22, 2021
2021 Dues Paid
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Old 04-28-2020, 09:03 PM
Mark H's Avatar
Mark H Mark H is offline
Join Date: Jul 2011
Location: Indianapolis, IN, USA
Posts: 97

I?ve been doing this as well. I am highly interested in your results, so please share your data. Thanks!
Mark Hansen
Indianapolis, IN
RV-7A N896PS - Flying as 10/19/2017
RV-10 Under Construction
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Old 04-28-2020, 09:06 PM
thiggins thiggins is offline
Join Date: Mar 2020
Location: Saluda,NC
Posts: 46

Back in the day, pulling the oil dipstick up and hooking it on the oil filler door was SOP for an A36 partnership we were in. Pretty much all the members did it, and it was darned obvious having the door propped open like that by the dipstick...missing that...well you shouldn't be flying if you didn't see it.

I was always amazed at the amount of steam rising out of the filler neck after a flight.
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