Old gasoline has exactly one legal destination: a household hazardous waste site or collection event, carried in an approved fuel container filled no more than about 95 percent. Mildly stale fuel can instead be diluted into fresh, around one part old to five parts new, but only after it passes inspection. Fuel that is dark, sour-smelling or showing a separate watery layer is beyond saving and damages any engine it enters. Nothing here ever goes down a drain, onto ground or into a bin.
Diagnose it the way a small engine shop does, because the springtime mower that will not start is this exact subject wearing work clothes.
The bench test
Pour a sample into a clean glass jar, hold it to the light, and read it like a mechanic.
| Fresh | Stale | Phase separated | |
|---|---|---|---|
| Measurement | What each state of fuel looks, smells and behaves like | Same | Same |
| Colour | Light straw, clear | Amber to brown, may be cloudy | Distinct layers, haze at the bottom |
| Smell | Sharp petrol | Sour, varnish-like | Same, often worse |
| Jar test | Uniform | Uniform but darkened | A watery layer under the fuel |
| Film | None | Brown varnish on the jar or dipstick | Varnish plus corrosion |
| Verdict | Use it | Dilute or dispose | Dispose, no exceptions |
Two different clocks age the fuel. Weathering evaporates the lightest compounds first, the very ones that make cold starting easy, leaving heavier fuel that ignites grudgingly. Oxidation builds the gums and varnish that coat carburettor jets shut.
Ethanol adds the third and worst clock.
The half-percent that ruins everything
Standard pump fuel is about a tenth ethanol, and ethanol drinks moisture from the air through every vent.
When absorbed water reaches roughly half of one percent by volume, the ethanol and water abandon the petrol together and sink as a corrosive layer. That is phase separation, and three facts about it settle most arguments:
- It is visible. The jar test shows the layer plainly, which is why the jar comes before any decision.
- It is irreversible. No stabiliser, shake or additive rejoins separated fuel. Stabilisers only slow the absorption beforehand.
- It is fast in humidity. Blended fuel can begin separating in as little as a month in damp conditions, which is why the practical life of pump fuel is one to three months against six or more for ethanol-free.
The separated layer corrodes steel and aluminium fuel systems, and the petrol floating above it has lost octane. Both halves of the jar are now the problem.
Three verdicts
Fresh enough: use it. Fuel a few months old that passes every line of the table runs fine, especially topped with fresh.
Merely stale: dilute it. Sound but darkened fuel can be blended away at about one part old to five parts fresh, ideally into a nearly full car tank that will be driven through quickly, or into forgiving older equipment. Small amounts, passed inspection, no layers, and never into a motorcycle, boat or anything carburetted and precious.
Dark, sour or layered: dispose of it. This fuel has one address, below, and every engine shortcut ends at a repair bill larger than the fuel was ever worth.
The one address, and the asymmetry worth noticing
Used motor oil enjoys a free national counter network, as our motor oil page maps, because used oil retains value as fuel and feedstock.
Old petrol has no such counter, anywhere, and the reason is the same economics running through this whole site in reverse: degraded, water-struck gasoline is worth nothing to anyone, so nobody builds a network to receive it. Household hazardous waste collection is the entire system. Municipal sites and periodic events take old fuel free, and that is the list.
Transport rules for the trip:
- An approved fuel container, sealed, filled to about 95 percent so vapour has room.
- Upright, ventilated boot, straight there.
- Mixed fuels declared: two-stroke mix, fuel with oil, anything uncertain, labelled and mentioned at the site.
- Rags and funnels that met the fuel travel with it as hazardous waste, not into the kitchen bin.
Ask whether the site returns your container; many drain and hand it back.
Preventing the next jar
- Buy small. A season's fuel, not a year's, is the single biggest fix.
- Stabilise at purchase, not at storage time, extending blended fuel from months to a year or more.
- Prefer ethanol-free for equipment that sleeps, where it is sold, and the jar test explains why.
- Run carburettors dry or close the fuel tap before winter.
- Store sealed, cool and shaded, since heat runs every clock above faster.
- Date the can with a marker, because memory is the least reliable additive.
Questions people actually ask
Can I just let it evaporate? No. Open evaporation is illegal fly-tipping into the air, a fire risk while it happens, and slower than the drive to the collection site.
Will a petrol station take it back? No. Stations sell fuel and are not licensed to receive waste fuel, which surprises people used to the oil counter next door.
Is old gas any good as a weed killer or fire starter? Both are genuinely dangerous folklore: soil and groundwater poisoning in the first case, flash burns in the second. The varnish smell is the fuel telling you it left usefulness behind.
How old is too old to dilute? Past a year unstabilised, or any age with layers, darkness or sourness. When the jar makes you hesitate, the hazardous waste site is the answer that cannot be wrong.
Common mistakes
- Trusting the mower to test it, using a carburettor as the jar.
- Shaking a separated can and hoping, against irreversible chemistry.
- Diluting into a nearly empty tank, the ratio backwards.
- Storing fuel in milk jugs and juice bottles, which petrol dissolves.
- Topping the container to the brim before a summer drive.
- Pouring it down the drain, one of the few disposal errors that reliably becomes an emergency.
Where this fits
The fluids that come out of engines and appliances before scrapping are on the fluids hub, the oil counter network that petrol conspicuously lacks is on motor oil recycling, and the pressurised cousins with their own rules are in the gas cylinder guide.
Frequently asked questions
How do I dispose of old gas?
Household hazardous waste collection is the only legal route: municipal sites and periodic events take old fuel free, carried in an approved sealed container filled to about 95 percent. Petrol stations do not take it back, and drains, ground and bins are never options.
How long does gasoline last?
Standard ethanol-blended pump fuel stays reliable for one to three months, ethanol-free for six or more, and stabilised fuel for a year or longer. In humid storage, blended fuel can begin phase separating in as little as a month.
How can I tell if gas has gone bad?
The jar test. Fresh fuel is light straw and smells sharply of petrol. Stale fuel darkens toward amber and smells sour or like varnish. A distinct watery layer at the bottom is phase separation, and that fuel is beyond saving.
What is phase separation?
Ethanol absorbs moisture from the air, and at roughly half a percent water by volume the ethanol and water sink out of the petrol as a corrosive layer. It is visible in a jar, irreversible by any additive, and it damages fuel systems while the fuel above loses octane.
Can old gas be mixed with new gas?
Only mildly stale fuel that passes inspection with no layers, at about one part old to five parts fresh, ideally into a nearly full car tank driven through quickly. Dark, sour or separated fuel never gets diluted into anything.
Why do stores take used oil but not old gas?
Economics. Used oil retains value as fuel and re-refining feedstock, so a free counter network exists to collect it. Degraded gasoline is worth nothing to anyone, so hazardous waste collection is the entire receiving system.
How should I transport old gas to the collection site?
In an approved fuel container, sealed, filled to about 95 percent so vapour can expand, upright in a ventilated boot, driven directly. Mixed or uncertain fuels get labelled and declared, and fuel-soaked rags travel as hazardous waste too.
Can I let old gas evaporate outside?
No. It is a fire hazard while it happens, an air pollution offence in principle, and slower than the drive to the site. Using it as weed killer or fire starter is more dangerous folklore still.
How do I stop gas going bad in the first place?
Buy a season's worth rather than a year's, add stabiliser at purchase, prefer ethanol-free for equipment that sits, run carburettors dry before storage, keep containers sealed and cool, and write the date on the can.
Is slightly old gas safe for my car?
Fuel a few months old that passes the jar test runs fine, particularly diluted with fresh in a near-full tank. Motorcycles, boats and small carburetted engines are far less forgiving, which is why they get only fresh fuel.