One major disadvantage of gasoline is that it doesn’t last forever and expires just like food. Sure, you can still run oil gasoline in a car or lawnmower, just like you can still eat expired yoghurt or cheese. But it naturally degrades and loses combustibility over time due to oxidation (exposure to oxygen) and evaporation of its volatile compounds. Gasoline usually lasts three to six months when stored correctly in a tightly sealed plastic container or metal tank.
However, the purity of the gas and the use of fuel stabilizers can shorten or lengthen its life. Most gasoline sold in western countries is “E10” gas made of 90% petroleum-based gas and 10% ethanol. Ethanol-blended gas usually has a shelf life of up to three months because of the high speed at which ethanol oxidizes. Because ethanol is hydrophilic (attracts water), it will also readily absorb any water or humidity in a sealed container resulting from condensation, causing both moisture contamination of the fuel and the eventual separation of the fuel into distinct gas and ethanol layers. Generally, the higher the ethanol content in the gas, the shorter its shelf life. E85 (85% ethanol) gas will expire sooner than E10 gas will.
Petroleum-based gasoline without any ethanol will still succumb to oxidation and volatile compound evaporation in a sealed container or tank. These processes usually occur more slowly in pure gasoline, so you can generally expect it to last at least six months when properly stored. Because pure gas is hydrophobic (repels water), it doesn’t absorb water or humidity as ethanol-blended gas does, which avoids moisture contamination and fuel separation issues. Fuel stabilizers are petroleum-based additives that you can mix in with gasoline before storing it to slow oxidation and volatile compound evaporation and extend the shelf life of the gas. Depending on the product, the stabilizer can increase gasoline shelf life by between one and three years. Stabilizers work best when you mix them with new gasoline. They are ineffective at slowing the degradation of old gas and can’t restore contaminated gasoline.
The easiest way to check the condition of gas is to pour a small amount of your stored gas and freshly pumped gas of the same type into two clear glasses and make a side-by-side comparison. If the gasoline is only slightly darker than the fresh gas or smells sour, it’s probably old and has lost its effectiveness but isn’t contaminated. If you observe separate layers of gas and ethanol in an ethanol-blended gas (generally, the gas layer will be darker and located above the lighter ethanol layer if the fuel has separated), or if the gas is significantly discoloured (milk chocolate or rust colour) or contains sediment or sludge, it has been contaminated by moisture or the solid by-products of oxidation.
Contaminated gas should never be used to power cars or equipment. It can promote corrosion or leave sludge or varnish deposits (a thin, transparent brown or orange film) on fuel lines, injectors, valves, cylinders etc. and can irreparably damage them. Instead, dispose of contaminated gasoline at a recycling facility.
Nils Willner
Nils is a Swiss-German engineer who is obsessed with old cars and engines. He is the author of "The Ultimate Classic Car Guide - How to Buy, Maintain & Repair Classic Cars" and the founder of EVC. His passion has always been with old cars and everything with wheels and engines.