When stored correctly, high quality gasolene should continue to be stable forever (well almost!). There are few factors that contribute to the degradation of petrol with the two primary concerns being oxidisation and water.
If petrol is not stored in an air tight container then the process of oxidation occurs. Fuel that has been exposed to air flow will begin to look cloudy and get darker in colour. Sometimes you may even be able to see particles floating in the fuel if it is badly oxidised.
Once the fuel is in this state it is dangerous to add it to an engine as the fuel will form deposits in the fuel system. Fuel stored in a vehicle is not air tight – particularly in motorcycles and therefore these deposits can build up and affect the proper working of the engine.
A common way to prevent this is to add anti-oxidants such as Lucas Fuel Stabilizer.
Water in the Fuel
Water in fuel is a big problem as internal combustion engines cannot ignite water! Water and petrol do not mix so if your fuel tank had transparent sides you would see the petrol sitting on top of a layer of water. This water is sometimes clear, but usually appears as a rusty or dirty colour.
The fuel pickup for most fuel pumps is located towards the bottom of the fuel tank so the engine will be fed the water first. Once the water gets through the fuel lines the engine will be starved of petrol and fail.
Many countries now have up to 10% ethanol (Brazil has 25%!) mixed into fuel for a variety of reasons such as the reduction of carbon emissions or to increase the octane rating of the fuel. Ethyl alcohols are hydroscopic and can easily absorb the moisture in air humidity and therefore increase the water content of the fuel.
This may not be such a big problem because the water that binds with the alcohol will be burnt by the engine, but it will reduce the quality of performance that can be achieved.
An old trick that can remove water from fuel is to throw a bottle of methylated spirits into the fuel tank. This trick exploits the hydroscopic properties of the alcohol so that water is absorbed into the alcohol and can ignite in the engine. However it is important to note that the alcohol will become saturated if there is more water in the tank than it can bind with.
A more controlled way of dealing with water in the fuel is to add a bottle of Wynns Dry Fuel or Wurth Petrol Engine Additive to the suspect fuel.
Other factors in fuel deterioration include contaminants such as rust, dirt or oil and the temperature. The degradation of fuel is accelerated by higher temperatures (above 26°C or 80°F) so fuel should be stored in cool areas and obviously out of direct sunlight!
Whilst not strictly stale fuel; poor starting using stored fuel could be attributed to government regulated volatility. I believe this is likely to only be a problem for the US.
This is measured using RVP (Reid vapour pressure) and differing levels are used depending on the ambient temperature. Fuels with a higher RVP evaporate more easily than those with a lower RVP rating because the components in the latter have a heavier molecular weight.
In summer a fuel of lower RVP (~7.8 to ~9 PSI) is used, which prevents the fuel from evaporating and being wasted into the atmosphere or causing vapour lock. Vapour lock is caused by the fuel turning into a gas in the fuel lines, which then starves the engine of fuel because the pump can only move liquid.
As winter comes around the RVP will be increased with as many as eight graduations and it could eventually end up in the region of ~15 PSI. This means that the fuel can evaporate more easily in colder temperatures therefore making it easier to start a vehicle.
The change in the fuel mixture or oxygenates is usually performed to improve the clean burning of fuels in the summer months to reduce smog and pollution. More information on this subject can be found on Environmental Protection Agency website.
This being the case you could find it difficult to start a vehicle in winter that is filled up with stored fuel from summer. One potential solution in this case is to get it started with some start spray or drain the fuel and replace it with the correct winter fuel.
Protecting fuel in storage
To prevent these problems from affecting your fuel there are five things you can do:
- Only use dedicated fuel containers such as jerry cans.
- Ensure containers are air tight and capped tightly to prevent evaporation and exposure to air and moisture.
- Use a fuel stabiliser product like Lucas Fuel Stabilizer.
- Fill containers as completely as possible leaving a 5% air gap for expansion if the temperature rises.
- Store containers out of direct sunlight where the temperature does not exceed 26°C or 80°F. If the temperature is exceeded then fuel will begin to degrade more quickly.
With these precautions in place some people have reported up to 5 years of stable storage, but I am sure there are others out there who have managed to keep fuel for longer.