Why does fuel go bad?

Dirty Fuel

Petrol spoils extremely quickly in Australian conditions due to elements such as light exposure, ambient temperature and time. In the worst cases, petrol can spoil within 30 days. Using contaminated or spoilt fuel can seriously damage your inverter generator’s engine. The fuel gums up and clogs vital components. Even with EFI models such as the AU4i and Redback spoiled fuel can cause issues at critical parts needed in the operation of an inverter generator such as the fuel lines, fuel pump and fuel filter.

Redback Inverter Generator

Your iTechworld generator can often sit idle in your garage or shed for months before a situation occurs where emergency power is needed. Quite often, it is when you need your generator the most that it will fail to start, and 90% of the time, this is caused by dirty fuel. To avoid such an event you should read the iTechworld inverter generator maintenance guide HERE.

 Why does fuel go bad?


The first thing that usually happens to stored fuel is the lighter chemicals inside evaporate, leaving behind a heavier, less volatile product. Petrol is the perfect fuel for small engines because it vaporises easily to form a combustible mix with air. Unfortunately though, when it sits unused its more volatile components evaporate, leading to poor engine performance. There is no simple way to test how much punch your fuel has lost, but you may notice your iTechworld generator running harder or louder than usual.


The second cause of fuel spoilage is oxidation, some of the hydrocarbons in the petrol will react with the oxygen to produce new molecules and peroxides, none of which are ideal to have in your engine or fuel tank. When oxidation occurs, you can easily tell by the odour of the fuel, as it will have a strong sour smell, similar to varnish or paint thinner. It is also noticeable visually, if you pour some of the fuel into a glass container, you will see that the fuel has turned dark and you may also notice small solid gum particles floating inside. Never ever use oxidised fuel in your iTechworld generator as the gum can clog your fuel filter, create deposits all through the fuel system, block up the injectors and generally give poor all-round performance. Peroxides can attack rubber and metal, stripping away the liner on fuel lines and pumps and attacking rubber hoses. This process can sometimes take a few months although it will occur faster if UV light can get into the container.


Finally, there is the problem of contamination. Water, is the main culprit in this situation as it gets into stored fuel containers via condensation as temperatures fluctuate. If the gas is relatively fresh, there are fuel additives on the market that combine with the water in the tank or engine to make it combustible (Available at your local auto shop). Another issue that arises from water contamination is bacteria and algae growth, but this isn’t as common.


How to keep stored fuel fresh

Always store your fuel ONLY in clean containers with airtight caps. In Australia, we can store fuel in steel or plastic containers, although steel containers can rust eventually. Keep the container close to full, to minimise exposure to air, but do not completely fill, as the petrol will expand and contract with changes in temperature. This being the case, always try to store your fuel in a cool, dry area to reduce evaporation and oxidation. If you are planning on storing fuel for longer than 30 days, consider adding a fuel stabiliser additive to your jerry cans.


What to do with spoilt fuel?

There is no easy way to get rid of bad fuel, but that is no excuse to just dump it down the drain or on the spare block down the road. If you can’t use it, contact your local council to ask where you can dispose of it legally. Some people recommend using it for your lawnmower, but you risk running into the same problems there instead. You are better off budgeting approximately how much fuel is required for each task, so you don’t need to store any. Fuel is expensive, there is no point having money tied up in something that could spoil.


How to avoid spoilt fuel

  • Only use fresh, clean petrol that is suitable for your generator (95 or 98 Octane).
  • To slow deterioration, keep petrol in an Australian-certified fuel container. If storing for an extended period (more than 30 days), drain fuel tank and carburettor dry.
  • If storing fuel in a container for longer than 30 days, use a fuel stabiliser additive.
  • Follow all suggested generator maintenance schedules as specified in your AU4i/AU2i or Redback generator manual.
  • Where possible, buy petrol from larger, busier service stations, as they are most likely to have fresh fuel.
  • Contact your local council to dispose of old or dirty petrol, as they often have days when you can drop off old/dirty fuel, paints, thinners, used batteries etc to the local rubbish tip or recycling station.


If you have any questions or feedback, leave them in the comments below!


Article author


Dave Petchell





I have a lot of small engin driven equipment that is stored for long periods between use.
It used to be to be a nightmare with crook fuel and the tarish that occurs and is deposited in the carby and injection stystem often resulting in a carby strip down to clean out gunge left by crook fuel.
However I have found there is a lot less issues if there is long periods between use if you simply add a small amout of outboard motor fuel oil to the tank of fuel
Upon shut down, turn the fuel off and allow the engine to run out of fuel
The outboard fuel oil will not harm the engine but will put a protective coating on carbureter or injection components which helps if there is a long period of time between use.
Most boat’s dont get used for months on end however pump some fuel in to the outboard and away they go.

Konrad de vos#2

The above suggestion does make sense! One thing you musty be aware of !….there is oil available for 2 stroke petrol engines, such as lawn mowers and outboard motors.
They need DIFFERENT types of oil! Most outboard motors are water-cooled ( except a few brands, produce air-cooled outboard motors), where as most lawnmowers are air-cooled.
When purchasing this type of oil , read the label to make sure it is suitable either for an air-cooled motor ( portable generators) or water-cooled 2 stroke engines.

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