How long does petrol keep, and what is E10 doing to my garden machinery?

Petrol has always gone off, but standard UK unleaded is now E10 — up to ten per cent ethanol — and ethanol makes two specific things worse for garden machinery. It attracts water, and it attacks certain rubbers and plastics used in older fuel systems. Neither matters much in a car that empties its tank every fortnight. In a mower that sits for six months, both matter a lot.

Safety

Step by step

1. Understand what actually goes wrong.Petrol loses its lighter, more volatile fractions to evaporation, which is what makes an engine hard to start. Separately, ethanol is hygroscopic — it pulls water vapour out of the air through the tank vent — and once enough water is absorbed the mixture separates, leaving a corrosive water-and-ethanol layer at the bottom of the tank where the fuel pickup is.
2. Know the realistic timescales.In a sealed metal container, petrol keeps reasonably for perhaps six months. In a vented plastic machine tank in a shed that warms every day, assume performance drops within a month and that it is a problem after three.
3. Buy small and buy often.A five-litre can bought in March and used by May is the whole answer for most households. Buying twenty litres because it is convenient is what creates the problem.
4. Check whether your machine is E10 compatible.Machinery made before roughly 2011 may have fuel lines, seals, floats or carburettor components that ethanol degrades. Older two-stroke equipment and vintage machines are particularly affected. If in doubt, use E5 — sold as super unleaded — which has a lower ethanol content.
5. Use a fuel stabiliser if it will sit.Added when you buy the fuel, not when you notice a problem, a stabiliser extends usable life substantially and is much cheaper than a carburettor clean. Follow the dosing on the bottle.
6. Consider alkylate fuel for occasional-use machines.Pre-mixed ethanol-free fuel sold for chainsaws and small engines keeps for years, burns cleaner and does not gum carburettors. It costs several times as much as pump petrol, which makes it sensible for a strimmer used twice a year and pointless for a mower used weekly.
7. Empty the tank before a long lay-up.Either run the machine dry, or drain the tank and then run it until it stops so the carburettor bowl empties too. An empty fuel system cannot varnish up. This is the single most effective habit.
8. Store fuel properly.In a container designed and marked for petrol, kept in a detached shed or garage rather than in the house, out of sunlight, away from any ignition source, and never below ground level. Keep it in the shade and as full as practical — a part-empty container has more air, and therefore more moisture, in it.
9. Dispose of old petrol correctly.Household waste recycling centres take it. Never pour it down a drain, onto the ground, or into a household bin, and never burn it off.
10. Get two-stroke mixes fresh each time.Mixed two-stroke fuel degrades faster than plain petrol because the oil separates as well. Mix what you need for the job rather than keeping a can of mixed fuel for months.

Tips

• Date the can with a marker pen when you fill it. Nobody remembers.
• If a machine has sat with fuel in it all winter, drain it before the first attempt to start rather than after four failed pulls.
• A clear jar and five minutes will tell you whether fuel has separated: cloudy or layered means dispose of it.

Common mistakes

• Storing machinery over winter with a full tank of pump petrol — The tank vent lets moist air in and out with every temperature change, the ethanol absorbs the water, and by spring the carburettor jets are varnished and the fuel has separated. It is the single most common cause of small-engine service bills.
• Keeping petrol in an unmarked or unsuitable container in the house — It is both a serious fire risk and against the domestic storage rules. Petrol vapour is heavier than air, flows along floors and can be ignited from a considerable distance by a pilot light, a switch or a fridge thermostat.

If it doesn't work

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