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Old 05-23-2022, 05:29 AM
WJaviation WJaviation is offline
Join Date: Jul 2020
Location: Pensacola
Posts: 61
Default Fuel Replace after 6 weeks?

My 12is has been sitting for about 6 weeks. Not run or flown in that time. its got about 14 gallons of 93 zero Ethanol fuel in the tank. Air temp has been 85-90 with 80% humidity during the days over that same time frame.

Should I drain the tank and replace with fresh? or just mix in some fresh 93 octane fuel and take her flying? or is there a more elegant way to check the gas is still good to go?
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Old 05-23-2022, 06:30 AM
Desert Rat Desert Rat is offline
Join Date: Aug 2018
Location: Wichita KS
Posts: 916

That seems pretty extreme. I don't know much about rotax engines, but I can't imagine any gas going bad enough that it would cause a problem in only 6 weeks.

If it were me, I'd just sump it really well, top it off and not worry about it.

personal opinion only. In this case I don't have anything to back it up with.
Terry Shortt
AGI, CFI, CFII, MEI, A&P, Janitor
RV7 Empennage & Wing done
Fuselage in process
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Old 05-23-2022, 07:01 AM
BoydBirchler BoydBirchler is offline
Join Date: Jul 2020
Location: Indianapolis
Posts: 167

I use 93 octane zero ethanol in my IO-360 and was told by the supplier, it is good for use for up to 5 years. I'm sure the fuel in my nurse tank has often exceeded 6 weeks.

The only caution is do not use winter mix fuel in summer weather. the blend used during winter has lower vapor pressure and is prone to vapor lock in warmer weather.
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Old 05-23-2022, 07:38 AM
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MacCool MacCool is offline
Join Date: Aug 2020
Location: central Minnesota
Posts: 1,099

I completely agree that 6 weeks is nothing....not enough time to cause significant octane loss. However, I had the impression that the OP might also be worried about condensation, therefore the possibility of water contamination. I think it's unlikely that there could be enough water to be capable of phase-separation, but that can be tested with a chemical test. I think that thorough attention to the sump drains on the tanks and any other low spot in the fuel system should demonstrate whether or not there's water there (let the sample stand for several minutes).
RV-9A, 2011, bought flying
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Old 05-23-2022, 07:39 AM
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Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,757

Originally Posted by BoydBirchler View Post
The only caution is do not use winter mix fuel in summer weather. the blend used during winter has lower vapor pressure and is prone to vapor lock in warmer weather.
Correct, but OP has fuel-injected 912 which runs higher fuel pressure. Should be OK with winter blend fuel. The real concern is the carbureted 912 which is low fuel pressure delivery. In-line electric pump and fuel return to the tank help, but no guarantee. RV-12 is tightly-cowled with minimal air flow on the ground. Several accidents can be attributed to vapor lock...
Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 790

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MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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Old 05-23-2022, 07:46 AM
PilotBrent PilotBrent is offline
Join Date: Dec 2011
Location: Hackettstown, NJ
Posts: 531

Seems like a good excuse to fly, ... would be shame to let all that good fuel to to waste!
Brent Connelly
Hackettstown, NJ
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Old 05-23-2022, 08:26 AM
Bob Y Bob Y is offline
Join Date: Jul 2017
Location: Piedmont, SC
Posts: 385

If you want to be more assured, top it off with 100LL and you’ll definitely be fine.
Bob Y - builder 12iS E-LSA
N569TT first flight 2/9/2020
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Old 05-23-2022, 09:26 AM
mbell mbell is offline
Join Date: Aug 2005
Posts: 165

At Rotax school we were told to drain carb bowls if they were going to sit more than three weeks. That was because of the fuel deteriorating and leaving deposits in the bowls. You don't have carb bowls so that's not an issue, but they also said mogas with ethanol can lose octane over time. I never have been able to determine what the limit on that time is, but the 100 HP Rotax is barely at it's octane limit with fresh premium unleaded anyway. For me, mogas in that ballpark of 6 weeks is drained and used in cars. Another reason to look forward to an unleaded 100LL replacement is it will be stable in storage.
Mike Bell
RV4-->6-->6A, (built by Villanueva, Tolle, and Mosher)
AirCam finished and flying!
RV-14A under construction.
Cameron Park, CA (O61)
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Old 05-23-2022, 09:42 AM
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MarkW MarkW is offline
Join Date: Sep 2011
Location: Edgewater, FL. X50
Posts: 1,195

Ethanol is alcohol which is hygroscopic. (Absorbs water)
Shelf life is short.
Ethanol free lasts much longer.
RV9 - N14MW - Flying
G3X - ECI Titan I0-320
Catto three blade prop
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Old 05-23-2022, 10:12 AM
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Snowflake Snowflake is offline
Join Date: Oct 2009
Location: Sidney, BC, Canada
Posts: 4,299

Living where it's common to put motorcycles away for 5-6 months at a time over winter, the procedure was always to fill the tank to the top, drain the carbs, and park it. Come spring, turn the fuel on, let the carbs fill, and fire it up. The fuel was always fine and the carbs never gummed up with this process.

Ethanol-enriched fuels absorb water from the air in the tank. They can't absorb water from air that isn't there... If it's full, there's (almost) no air. Even a vented tank won't circulate anything of significance through the vented cap in 6 months... 6 weeks is nothing.
Rob Prior
1996 RV-6 "Tweety" C-FRBP (formerly N196RV)
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