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I have my bike (bought in mid-2019) sitting idle for over a year due to covid, as I went back to my hometown and left my bike in my office parking.

Before leaving I refueled by bike with premium fuel all the way to the top to stop any rusting in the tank. I had a talk with a few people and they told me that the petrol in my bike needs to be drained before starting it as the fuel might have expired and it might not be good for the engine. I don't want to waste over 12 liters of petrol and I never thought that something like petrol can expire. Is it true? Do I really need to throw away all that petrol? Does petrol sitting in a tank can expire?

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    Well if you haven’t sorted the battery yet, then you have time to clean the fuel system.
    – Solar Mike
    Aug 8, 2021 at 16:34
  • @SolarMike yeah, I am just collecting information on how to fix it so I won't get overcharged on unnecessary stuff from the mechanic. I am yet to go back, maybe in a few weeks. Aug 8, 2021 at 19:04
  • I wonder if the petrol itself or the additives go bad first. Might be a good detail for someone who is writing an answer to include. Aug 9, 2021 at 15:07
  • My wife almost never drives her Chevy Volt (plugin hybrid) far enough to get it to switch from battery to gas. Recently it warned her that the gas in her tank was old. In this situation it starts running on gas in an effort to burn off the old fuel.
    – user19542
    Aug 9, 2021 at 23:02

4 Answers 4

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Petrol fuel aging modes:

  1. Evaporation. Depends on temperature and the availability of ventilation. The fuel is a mixture of components that evaporate at different rate. A fuel depleted of volatile fractions may refuse to start burning in the engine. It may as well completely disappear or leave a small amount of oily substance at the bottom.

  2. Polymerization. Depends on temperature, availability of oxygen and especially on the fuel quality. Light also helps if the tank, the tubing or something else is transparent. Small molecules combine, sometimes all the way up to creating a glass-like substance (the evaporation helps to remove the still liquid components). It may clog to death the small openings in the carburettor / injector / pump / whatever and even require time-consuming cleanong or even replacement of the clogged parts.

  3. Development of microorganisms. Depends on the bio-content (etanol, etc), but to some extent it happens even in completely crude oil derived substances. Also depends on the temperature, availability of water (condensate) and air. Most bacteria develop at the interface between the water droplets, fuel mass and air.

Bacteria leaves behind acids. Acids promote corrosion of the tank. Corrosion products (rust), combined with polymerization products acting as a glue and the bacteria colonies themselves make together a very good clogging agent.

2-stroke oil mixed in promotes both polymerization and bacteria development.


How to start a long-stored vehicle:

Try syphoning some fuel from the bottom of the tank into a transparent container first.

  • Clear, transparent, colorless or somewhat yellow (or whatever the original fuel color was), runny liquid of the expected amount = good.

May be mixed 1:1 with fresh fuel for better results.

  • Opaque, muddy, brownish, viscous liquid, rust particles, fuel way less than what was left = bad.

Fuel system cleaning required. Check for leaks or advanced corrosion in the tank.

2T oil also degrades, usually faster than the gasoline it is mixed in. If your bike is 2T, think about adding fresh oil. It is better to run the engine with twice the oil rather than with no real lubrication.

Be aware that 4-stroke motor oil degrades as well, by all of the above mechanisms. An oil change is highly advisable if the engine is stored for a year.

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It depends a lot on how well the fuel system was sealed. The most likely problem is that more volatile compounds will evaporate first, changing the properties of the gasoline. I once left a motorcycle a couple years in a warm climate and had not shut the tank valve. When I opened the carb floats, they were full of a clear yellow gel with no odor and required complete cleaning. On the other hand I have a Honda portable generator that sits for two years , then will start with a couple pulls; when it has been properly shut down. So you should be good if there has not been evaporation.

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    Presumably a portable generator will be designed with more thought to the possibily that it may sit unused for years on end and still must work reliably when needed (which may well be an emergency situation!), than a motorbike. Aug 9, 2021 at 10:42
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    Also, no light, low temperature, little oxygen. But even in a properly sealed opaque container in room temperature storage, petrol only has a shelf-life of a year or so. That doesn't mean you're guaranteed it will have gone bad over the year in storage, but yes, petrol definitely does decay. It's a highly refined product with a lot of easily accessible energy - it would be weird if it didn't decay, even in decent storage.
    – Luaan
    Aug 9, 2021 at 10:42
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    @leftaroundabout Also, is it a diesel generator? Diesel has a considerably longer shelf-life, and diesel engines in general tend to be less picky about the fuel quality than petrol engines (at least until you get to modern "eco" diesel systems, but you won't find those in portable generators anyway).
    – Luaan
    Aug 9, 2021 at 10:43
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Yes, petrol/gasoline sitting in a tank will absolutely expire. Gasoline is not a single compound, it is a blend of hydrocarbons, some of which are heavier (longer chains) and some of which are lighter (shorter chains). Over time the light fractions evaporate, leaving the heavier ones behind. Reactions with oxygen can also slowly polymerize the hydrocarbons, further thickening the fuel and impacting its ability to vapourize and burn. This heavier fuel can burn less cleanly, leaving gummy varnish-like deposits in the engine, gumming up filters, etc.

Fuels also change depending on the time of year and the location. In places with cold weather the fuel at the pumps in winter is produced with more light fractions to assist with evaporation and ignition in colder weather. This "winter fuel" loses its light fractions much faster once warm weather comes and tends to be the type of fuel that expires most quickly. In hotter climates and in summer the fuel tends to be heavier out of the pump and expires less quickly, but in all cases old gas is a recipe for fouling up filters, injectors, and the engine itself.

Premium fuel is less likely to contain ethanol, but many still do. As noted in other answers, this also can take up water or evaporate itself, giving another mechanism that ages and fouls the fuel over long periods of time.

Just toss it and fill with fresh fuel. I always empty the tanks of any seasonal equipment that is going into storage until the next year and refill with fresh. Any fuel that doesn't get used that season just gets siphoned out and put into the cars (which will happily use it up). The engines are then run after removing the fuel until they stall from complete fuel exhaustion to make sure no old fuel is left in the lines, filters, etc. Best to not store your equipment with any fuel in it at all.

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If this were mine, I'd drain the tank completely and properly dispose of the old fuel. Gasoline, especially ethanol containing fuels, do not age well. When they sit moisture in the air will dissolve into the fuel. That then combines with the ethanol and takes it out of solution with the gasoline. Since it's heavier than gasoline it falls to the bottom of the tank.

Now you have a layer of water/ethanol at the bottom and gasoline at the top. The water/ethanol mix is quite corrosive and will attack metal, plastic, and rubber parts.

Further, this mix tends to create "varnish" which will clog lines, tubes, orifices, etc.

Is it worth it to try to save 12 litres of fuel at the risk of expensive repairs? I think not.

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    Ethanol content is really the key. If it doesn't have ethanol in it, it can remain usable for many years. It's the ethanol which absorbs the water. Seeing as how the OP filled the tank completely full, the fuel may still be usable, because there'd be less air in it to absorb water from. Aug 8, 2021 at 17:14
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    Could you also drain the fuel into a canister, mix it with fresh fuel to some extent and keep using it?
    – Tim
    Aug 9, 2021 at 11:08

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