How long before the gas goes stale?

[Duke W.]

This is an oft-asked question among vintage car owners.

Most gasolines are blended with stabilizers and are claimed to have a "shelf life" of about a year, and I think this is conservative. Based on this and other research over the years my recommendation is that vintage cars should be driven enough every year to consume at least a full tank of fuel. So for a typical vintage Corvette that gets low to mid-teens MPG around town that's about 200-250 miles, and for cars that are stored for a few winter months the tank should be filled just prior to storage. This minimizes the vapor dome at the top of the tank, which will minimize the amount of moisture that the ethanol laced fuel might absorb from the air.

This is especially important on cars with vented fuel systems. Since they "breathe", any moisture that's absorbed from incoming air as ambient air pressure drops drawing air into the tank and temperatures drop below the dew point, is there until it's consumed with the fuel or drops out to the bottom of the tank if the amount exceeds the co-solvent capacity of E10 gasoline.

The current issue (May/June 2024) of Car and Driver has an interesting article on plug-in hybrids, and a sidebar at the bottom of page 63 reads as follows:

"Won't the fuel go bad? If the thought of fuel sitting in the tank unused for an extended period has you reflexively reaching for a bottle of Sta-Bil, calm down. The Prius Prime owner's manual states that to keep the gas fresh, the engine needs to burn only half a tank per year - that's 5.3 gallons good for about 275 miles of hybrid running. If the owner fails to meet this threshold, a warning light comes on". [How clever!]

Since all modern fuel systems are essentially "sealed" with vacuum relief valves that are not likely to open unless the car is in the eye of a hurricane or tornado, I'll stick with my original recommendation as above for vintage cars with vented fuel systems.

Duke

[Alexander D.]

Re: How long before the gas goes stale?

Thanks Duke for this interesting and useful info!

[John S.]

Re: How long before the gas goes stale?

Duke, very interesting. Does your recommendation change for cars running non-ethanol ??

[Duke W.]

Re: How long before the gas goes stale?

I stick with that recommendation for either straight gasoline or E10, especially on cars with vented fuel systems. You can probably about double that and do no harm for modern cars with "sealed" fuel systems, assuming the car is stored inside and not subject to condensing humidity or extremely high temperatures for any length of time. It's a subjective call.

In terms of water contamination, E10 acts as a co-solvent allowing a greater percentage of water to be homogeneously absorbed before saturation at which point a water-ethanol blend separates and sinks to the bottom of the tank since it's denser than the E10 blend. So in a way ethanol may actually protect the fuel system because the amount of water straight gasoline can absorb (less than one percent) is much less than E10. It's my understanding that water won't harm the fuel system until it separates, which would be water for straight gasoline and a water-ethanol mixture with E10.

Circa 1965 my SWC had a no start condition during a Seattle cold snap when ambient temperatures remained below freezing for a few days. I suspected frozen water in the tank, and sure enough, after it warmed up the engine started. I did the normal thing...add a can of HEET to the tank, which was basically denatured alcohol that caused the water to be absorbed by the gasoline and consumed in combustion. I never experienced that problem again.

The other way gasoline degrades is oxidation that can lead to the formation of "gum and varnish". It's very slow prrocess and, actually the colder the temperature, the slower it goes, which is why I consider additional "stabilizer" during winter storage to be unnecessary since the fuel already has stabilizer additives added by the refinery.

For the past 30 years I've stored cars, indoors, for up to eighteen month without additional stabilizer and have never had any fuel system issues; and since I live near the coast in Southern California, ambient temperatures rarely get below 45F or over 80F.

Duke

[Bruce M.]

Re: How long before the gas goes stale?

My 66 is garaged every summer in Florida with a dehumidifier struggling to keep the air dry. The original gas tank shows no rust after doing this for ten years and using Stabil, but the electric bill begged for an alternative. A 3-car garage with 3 overhead doors is far from airtight.

I considered a car bag with desiccant but can't do that and raise the car on blocks in case of flooding. So, I built my own "bag" by draping plastic over the car and taping it to the epoxy painted floor. Two buckets of Damp Rid went under the car prior to taping the plastic to the floor. The desiccant claims to be effective for 6 months and each bucket absorbs about half a gallon of water.

Every year I leave no more than 1/4 tank of gas in case a leak develops. Does anyone see a flaw in this approach to humidity control and moisture accumulation in the tank? The car cover did go on first to prevent paint damage.

[Larry E.]

Re: How long before the gas goes stale?

[QUOTE=Bruce Maser (59403);948642]

I considered a car bag with desiccant but can't do that and raise the car on blocks in case of flooding. So, I built my own "bag" by draping plastic over the car and taping it to the epoxy painted floor. Two buckets of Damp Rid went under the car prior to taping the plastic to the floor. The desiccant claims to be effective for 6 months and each bucket absorbs about half a gallon of water.

Bruce: I have been using "Car Bags": for over 30 years with great success. I'm in Illinois with nice "unheated" no
electricity building with concrete floor. At times I have left the vehicle (Corvettes/Camaros) in the bag for up to
3 years WITH NO ILL EFFECT. I just do not think you can get the same effect with "taping" your bag to the floor.
It will not give you the same sealed effect as with a solid bag. JMHO BTW> The liquid the desiccant draws is not all
water and is very caustic when taken out. I assume you take the battery out of the car as instructed to do. I would never
do what I do without using straight 100LL Av. Gas. as it will stay fresh/usable for SO MUCH MORE THEN THAT of
PUMP GAS THAT IS AVAILABLE REGARDLESS IF IT HAS ETHANOL OR NOT. Thanks;Larry P.S. If you do a search
on this board on "stale gas" you will find out that there are reports of airplanes that have not been used for 15 or more
years AND THEN THE AV GAS IS STILL TESTED GOOD AND THE AIRPLANE STARTS!

[Owen L.]

Re: How long before the gas goes stale?

This is from a Sunoco page:

There’s another advantage of higher-octane fuels: They’re more stable when it comes to storage. To explain the science behind that fact, we’re going to crib from a post on the Sunoco Race Fuels website by Technical Specialist Zachary Santner:


“87-octane fuels tend to be less refined and contain more unstable hydrocarbons. As the months pass during storage, these unstable components react to form gums, varnishes and lower-octane hydrocarbons. As a result, the octane can decrease within months for 87-octane fuels, especially when stored under less-than-ideal conditions.


“93-octane fuels are more refined and contain more stable hydrocarbons. These stable hydrocarbons can last two to three times longer than those in 87-octane fuel. Even with proper storage, 87-octane gas can start to degrade in three months; 93-octane fuel should last closer to 9 months before degradation is noticeable. Keep in mind that 93-octane fuels are still susceptible to octane loss and vapor pressure decreases due to butane evaporation.”