People always say “fuel goes bad,” but that phrase is almost useless without explaining what “bad” actually means or how fast it happens. It’s not a light-switch event where fuel is fine one day and engine-killing the next; it’s a spectrum of chemical changes that different machines tolerate differently. There are basically two major failure modes:

1.loss of volatility / octane and formation of gums and varnish, and

1. water uptake, phase separation, microbial growth, and corrosion (especially in ethanol blends and diesel).

Those don’t happen on identical timelines. Volatility loss and gum formation start first: you can see effects in a few months depending on conditions. Ethanol blends (E10 etc.) absorb water, oxidize faster, and are a lot more sensitive. Diesel has its own slow-burning issues like microbial sludge and filter-clogging.

The timeline is gradual, not a cliff. For regular gasoline with no ethanol and kept sealed in a cool place, you generally get something like 6–12 months before it’s “seriously degraded.” With ethanol blends, you can start seeing problems in as little as 1–3 months, especially in small carbureted engines. Diesel can go 6–12 months untreated, sometimes a year or two with proper additives and biocide. But again, these are broad windows: storage temperature, oxygen exposure, sunlight, and container material matter a ton.

And that brings up the second important point: not all machines respond the same way. Small carbureted engines (lawnmowers, chainsaws, generators, motorcycles) are by far the most vulnerable. They have tiny jets and bowls that gum up easily, sit unused for long periods, and don’t have sealed fuel systems. They can fail on fuel that a modern car would burn without much complaint. Modern fuel-injected cars and trucks are way more tolerant because their tanks are sealed and their injectors can deal with slight volatility loss better. Diesel engines fail in a different way entirely: usually through clogged filters and microbial deposits rather than straight “won’t start” behavior.

People ask why we don’t have extremely specific controlled data like “90-octane fuel stored at 16–18 °C in a 10L plastic can failed to start a 2009 Toyota after 4 years but a lawnmower after 11.” And the short answer is: the studies do exist, but they’re fragmented and focused on particular niches. Government and industry labs do accelerated stability tests (ASTM methods), the military has reports on how long diesel and jet fuel can be stored, NREL/DOE has done real-world testing on ethanol blends in small engines, etc. But nobody has produced one big consumer-friendly chart crossing every fuel type, temperature, storage method, and machine model.

So the real picture is: fuel degradation is gradual, multi-factor, and highly dependent on both the fuel formulation and the engine design. Cars tolerate stale fuel for a surprisingly long time, small engines don’t, diesel has its own microbial clock, and ethanol blends shorten the safe window dramatically. It’d be great to have a big pharmaceutical-style government shelf-life study for hydrocarbons, but right now all we have are scattered but consistent findings: fuel absolutely degrades, but the speed and consequences vary a lot depending on what it’s stored in and what you’re trying to run it in.