Stored Gasoline and Octane

[Shel_B]

Here's the scenario: there's gasoline in the vehicle's tank and the vehicle is not driven for about three-four months or so, or the vehicle is driven very little that the gas from the previous fill remains in the tank for that 3 - 4 month period. Will the gasoline degrade during that time, and specifically, will it lose octane?

 

[Kira]

No it won't.
I say that from a position of impure ignorance.
1) Many posts here have regarded starting engines which have sat for a longer time.
2) Fuel sitting in the sealed atmosphere of a modern fuel tank won't ever be exposed to enough oxygen to oxidize anything.
3) I never heard of octane molecules simply changing with time.

Thank you BITOG for making me impure.

 

[atikovi]

I've run cars on gas that was 5 to 8 years old and you couldn't tell the difference from fresh gas.

 

[JohnnyG]

As long as it's fairly well sealed from evaporation, it probably won't.

However, GM felt that it was a good idea to program a fuel monitor logarithm in the Volt. If the monitor determines that the engine hadn't run for 9 months to a year, a message would come on the dash stating "Fuel Maintenance" , and the engine would start up. This was probably due to additive longevity and making sure injectors and fuel system components were sitting with fresh fuel.

 

[kschachn]

Lots of misconceptions about octane out there. Octane is a hydrocarbon molecule with eight carbons in the backbone, but there are several isomers. The octane molecule (whichever isomer) is a stable molecule. Each isomer has a different octane rating however, as that is a different test and different property than the definition of "octane".

Besides isomers of the octane molecule there are other ways of increasing the octane rating of gasoline. Commercial gasoline is a broad mixture of different compounds, and sometimes a more volatile component can be used to give a higher octane rating to the mixture. Compounds with a higher vapor pressure can preferentially evaporate from the gasoline if there is an opportunity to do so. So yes, over time it is possible for the rating to change but it would depend on the length of time and the storage conditions, as well as the specific formulation of the fuel. But three to four months in a sealed tank with a properly functioning vapor recovery system is not going to do it.

 

[Nick1994]

I've run cars on gas that was 5 to 8 years old and you couldn't tell the difference from fresh gas.

It must depend where you're at. I maintain my aunt's '70 Beetle and it's always run great. We let it sit for a year once, 91 octane Chevron gas, and it ran horribly after that even after a tank of fresh gas. Took it to a VW shop and they had to rebuild the carburetor. Since then I always use Stabil and try to drive it more.

Had the exact same problem with my '05 Honda ATV.

 

[Kira]

Took it to a VW shop and they had to rebuild the carburetor.

Even though every vehicle guy says it's no good to idle a car for maintenance (think cheap used car dealer lots with all the hoods up because all the batteries were dead etc.), I believe there is some good from firing up an engine at least once every 4 months just to keep the juices flowing. In the case of carbureted engines, it keeps the float from sticking. You can change the oil when the poor car sells.

 

[SC Maintenance]

The "official" stance from the refiners is gasoline meets spec for 90 days from production with ethanol and 180 days from production ethanol free. Because as much better described above, the octane level will degrade.

However there is likely some safety factor in those numbers, and if your car is modern / EFI closed loop - it will run on lower octane fine likely - it will just retard the timing and your mileage will decrease.

I would be more worried about ethanol settling out of suspension than octane degradation personally.

If your planning on leaving it sit - then use a fuel conditioner, just because its cheap insurance. If it simply ends up sitting that long, I wouldn't worry about it.

 

[Nick1994]

Even though every vehicle guy says it's no good to idle a car for maintenance (think cheap used car dealer lots with all the hoods up because all the batteries were dead etc.), I believe there is some good from firing up an engine at least once every 4 months just to keep the juices flowing. In the case of carbureted engines, it keeps the float from sticking. You can change the oil when the poor car sells.

Well, I actually did start it every 3-4 months. The reason it sat without driving was the bracket for the steering column broke, and with other stuff going on at the time and then insane summer heat, it got put on the back burner. The battery was fine sitting all that time though.

 

[benjy]

all my carb'd engines get NO EATHANOL gas, others are used enough + get fresh gas. VP notes their NON EATHANOL gas os good for YEARS!!! but of course $$$ a few locals have pump eathanol + its spendy as well!!

 

[odie]

Always us 90 non E throughout the year(s) . Wish it was still Top Tier . Kept in plastic gas cans .

 

[dogememe]

As long as it's fairly well sealed from evaporation, it probably won't.

However, GM felt that it was a good idea to program a fuel monitor logarithm in the Volt. If the monitor determines that the engine hadn't run for 9 months to a year, a message would come on the dash stating "Fuel Maintenance" , and the engine would start up. This was probably due to additive longevity and making sure injectors and fuel system components were sitting with fresh fuel.

And they call for premium. Why? I have no idea. You’d think it would be better to require regular gas as it cycles quicker at the gas stations and thus is probably fresher to start with?

 

[WeyounDS9]

Here's the scenario: there's gasoline in the vehicle's tank and the vehicle is not driven for about three-four months or so, or the vehicle is driven very little that the gas from the previous fill remains in the tank for that 3 - 4 month period. Will the gasoline degrade during that time, and specifically, will it lose octane?

Yes it will, but at 3 or 4 months perhaps only incrementally.
If its ethanol containing fuel, like most are these days, the ethanol phase might pool at the bottom of the tank and it may be a bit heavy in water.
No fuel system is completely tight btu todays cars are better about in than in days past.

So in summary, yes u may lose some Octane rating due to the evaporation of light fractions, but nothing crazy.
"Octane" in your fuel is not the actual molecule but simply a measurement on how close your fuel behaves during combustion to that reference molecule.
Ethanol phase seperation may occur,

Source: Chemist.

 

[kschachn]

all my carb'd engines get NO EATHANOL gas, others are used enough + get fresh gas. VP notes their NON EATHANOL gas os good for YEARS!!! but of course $$$ a few locals have pump eathanol + its spendy as well!!

Darn EATHANOL.

 

[atikovi]

It must depend where you're at. I maintain my aunt's '70 Beetle and it's always run great. We let it sit for a year once, 91 octane Chevron gas, and it ran horribly after that even after a tank of fresh gas.

Old cars have carbs with vented fuel bowls and gas tanks. The fuel oxidizes much faster than sealed fuel injection systems.

 

[y_p_w]

There are worse things than reduction in octane rating from storage. But my understanding (like others) is that modern sealed fuel systems don't oxidize much and ethanol doesn't break down much either. But a fuel can is another matter.

 

[JohnnyG]

And they call for premium. Why? I have no idea. You’d think it would be better to require regular gas as it cycles quicker at the gas stations and thus is probably fresher to start with?

Yes, my 2012 Volt called for premium fuel. I imagine it was for the added detergents back then. They knew that if people used those cars as intended, they wouldn't be burning much gasoline.

 

[dogememe]

Yes, my 2012 Volt called for premium fuel. I imagine it was for the added detergents back then. They knew that if people used those cars as intended, they wouldn't be burning much gasoline.

Did you run regular or premium? I’m sure the dealer put regular in it and it seemed to run about the same. Then again I do use it as a hybrid most of the time so I’m not leaving the gas to sit for months.

 

[JohnnyG]

Did you run regular or premium? I’m sure the dealer put regular in it and it seemed to run about the same. Then again I do use it as a hybrid most of the time so I’m not leaving the gas to sit for months.

Yeah, most people never quite figured out their intended use back then, most never plugged them in. I plugged mine in nightly, dash showed about 250 MPG, which it was as high as it would go. I DID see the fuel maintenance cycle kick in twice during my lease.

These cars were pretty advanced back then and people just didn't know what a hybrid was. They all thought "PRIUS" when someone said hybrid! Toyota had a better advertising campaign.

 

[Revving2Redline]

Always us 90 non E throughout the year(s) . Wish it was still Top Tier . Kept in plastic gas cans .

Oh my guess is if you had a couple capsules of the red line SI 1 it will be superior to the detergents you get at a gas pump. I've had the misfortune of having to use clear fuel that's not high detergent and that's all I do is add the red line.