Fuel Replace after 6 weeks?

[WJaviation]

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?

[Desert Rat]

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.

[BoydBirchler]

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.

[MacCool]

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).

[Piper J3]

Originally Posted by BoydBirchler

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...

[PilotBrent]

Seems like a good excuse to fly, ... would be shame to let all that good fuel to to waste!

[Bob Y]

If you want to be more assured, top it off with 100LL and you’ll definitely be fine.

[mbell]

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.

[MarkW]

Ethanol is alcohol which is hygroscopic. (Absorbs water)
Shelf life is short.
Ethanol free lasts much longer.

[Snowflake]

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.