The Truth About Bad Fuel

All sorts of problems are blamed on petrol that's 'gone off' either in a motorcycle's fuel tank, separate petrol can and even rumors of bad petrol at not so busy petrol stations. Is this really a problem and, if so, how should you combat it?

So we called a well known English-Dutch petrol company and talked to one of there research chemist to cut through the hype and give us the facts on bad petrol.

'Petrol in a sealed can does not go off, provided it is container made to hold petrol. Petrol stored at petrol tank underground storage tanks will be okay for years. About the only additive in fuel is the detergent, and it doesn't evaporate. In these lead-free days, octane number depends on the molecular make-up of the fuel, and all you need to do is stop evaporation. Even the smallest molecules in there (such as hexane) can't get through metal. Plastic, as used for fuel containers, is itself made up of long-chain hydrocarbons (usually high-density polyethylene) which have similarities with the short-chain petrol hydrocarbons, so small petrol molecules worm their way into the relatively large spaces between the plastic molecules and get out the other side. Losses can be several percent over long periods (months) but thick plastic helps reduce this. You can't beat metal with its compact atomic structure for storing petrol. This is the main reason why underground petrol storage tanks are made of metal.
Petrol is okay in a can with an airspace; a tiny amount of the more volatile fractions of the fuel will evaporate to fill the airspace, and that's the end of it. It becomes a 'closed system in equilibrium'. Molecules go back into the liquid phase at the same rate as they leave it and enter the gaseous phase. Even a three-liter airspace in a five-liter can will only contain a very small amount of fuel vapor, equivalent to one or two cc's of fuel depending on the temperature.

As for a vented fuel tank like most motorcycles have fitted, there will be some minor vapor loss of the more volatile fractions, such as hexane, but nowhere near as bad as losses from a container exposing a large surface area of fuel to the air – such as a frying pan. But, then, if you keep your petrol in a frying pan, you're asking for trouble anyway! Volume changes in the tank airspace due to daily temperature changes cause the tank to 'breathe' but one breath every 24 hours isn't so bad. Even a few months of this with a half full fuel tank isn't a big deal. I get the impression from some motorcycle owners that they think the fuel tank gasps away like a bull elephant running a few hours... In reality, it's more like a hibernating mouse.

As for petrol evaporating from the carburetors, some fuel, particularly from non-mainstream suppliers, can leave a tacky deposit when it dries out, but this was more of a '90s problem. We, as in the oil industry, don't hear of it happening today. Even so, it's a good idea when putting a motorcycle in storage to add a drop of good two-stroke oil to the fuel, run the engine until it's good and hot, shut down and cover with a blanket (to isolate temperature and airflow fluctuation). Then, if the fuel dries out, a protective oily film will be there which is easily washed off when the motorcycle is run again.
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