Dealing with Bad Fuel

Marco Antonello — 3 February 2015
Tips and How-To

Do you ever think about the fuel you are putting in your tank? Beyond worrying about the price and which grade of fuel to buy, probably not. But you should.

A bad batch of fuel can wreak havoc on both diesel and petrol engines, especially common rail diesel engines, with repair bills in the thousands of dollars.

So let’s have a closer look at the how, where and why of fuel contamination.


Fuel quality in Australia is generally not as good as some other parts of the world, and the main contaminants are water, algae and sediment from the bottom of the tanks at the service station. Contamination is a consequence of poor management and, unfortunately, there isn’t much we can do to prevent ourselves from filling up our next tank with contaminated fuel.

In every litre of fuel there is always a nominal amount of water — the Australian standard determination is 0.05 per cent by volume. This amount of water is said not to impact fuel system operation and makes its way into the fuel via condensation in the tank and by accidental ingress when tanks are filled.

Airborne and waterborne spores can also contaminate fuel. Different types of spores require different conditions to germinate and they will only germinate in the right conditions. The spores settle on the surface of the fuel where they remain dormant, then when movement washes condensation off the sides of the tank, these spores get mixed into the fuel in the process.

Dissolved oxygen in the water acts as a trigger for germination and heat accelerates the process. In warmer climates, such as the outback, more rapid spore growth is experienced because of the higher temperatures and also because a lot of outback tanks are above ground.

More causes of contamination

Another cause of contaminated fuel is sediment in the bottom of the fuel tanks. Suction from underground storage tanks to the dispenser is via a tube that is approximately 50mm from the bottom of the tank. This 50mm allows condensation, ground water ingress and flash rust to accumulate.

In the meantime, tankers filling the tank at a rate of 100L/minute via a 100mm drop tube will be agitating and emulsifying tank-bottom debris. So, when the tank is filled by the fuel tanker, contaminants mix with the fuel being pumped into your tank.

Diesel engines are especially susceptible to contaminated fuel because the pumps and injectors have only very fine tolerances. Petrol engines can also be affected by dirty fuel, which can cause loss of power and detonation (also known as pinging), and can clog up injectors, which can lead to expensive repair costs.

Effects of contamination

If you are a victim of contaminated fuel, you will experience a rough idle, poor performance and increased fuel consumption. Unfortunately, if these symptoms start to show, damage might already been done. If you are driving a petrol or early diesel (pre-electronic injection), hopefully all you will need to do is flush the fuel system, which should include the diesel pump being drained, the fuel filter replaced and the tank filled with clean fuel.

Newer, common rail diesel engines aren’t as lucky. If spores grow into algae, the fine tolerances in both the pump and injectors means these parts of the fuel system will clog and cause up to $15,000 in repairs, which are not covered under a new car warranty.


There are a few fuel treatment products on the market, mainly for diesel engines, that help prevent the growth of algae and bacteria during fuel storage and also prevent gum build-up. A few brands available are Chemtech Diesel Power, Fuel Doctors Australia and Wynns Spitfire Diesel Treatment. These products will help clean your fuel system, but are only preventative measures and will not actually remove algae.

Another preventative step is to install an aftermarket water trap as well as the original fuel filter. This will give you a glass fuel bowl and another point in the system where water and contaminants can be trapped.

Dealing with fuel contamination is an exercise in prevention because if our vehicles are already showing signs of having contaminated fuel on board, it is usually too late. Thousands of dollars damage can be done without you even knowing — so prevention is the key.

Using a fuel treatment of some sort can help prevent airborne or waterborne spores from growing into algae, and will also clean our fuel system as we drive. But before you go out and buy one, it is best you speak to your mechanic to see if they have had experience in dealing with these issues, so they can recommend a suitable treatment.

Being careful where we buy our fuel is another great way of prevention. This can be easy if you are filling up at the same service station each week, but is almost impossible when travelling on the road.


If you believe you have a bad tank of fuel, the best solution is to drain your fuel system. This means first of all draining the fuel tank or tanks in your vehicle. It is very depressing considering how much a litre the damn stuff is worth, but it is the best way. Most tanks have a drain plug, which makes it relatively easy, but it might still need to be removed to get 100 per cent of it out.

Once the tank is empty, methylated spirits can be used to dry out any excess water that might still be in the tank. I have heard of people going to the extent of cleaning out the fuel lines, but I don’t think it’s necessary.

The next thing is to replace the fuel filter. Hopefully, the filter has done its job and hasn’t let anything other than fuel through, but if there are contaminants in the fuel system they can clog up the filter, so it is also good practice to check and replace the fuel filter after the new tank of clean fuel has been used, just to make sure. If you have a petrol vehicle, this is all you need to do.

If you have a diesel, then the diesel pump can also be drained. This can be done via a drain plug on the side of most pumps, but make sure you’re undoing the right bolt.

Once the tank is back in and full of fuel, the system will need to be primed. In a diesel vehicle, this is done with the hand pump on the top of the fuel filter and this is how it’s done in the majority of 4WDs in Australia. For a petrol vehicle, it is just a matter of turning the engine over until the fuel is pumped into the engine.


Here are a few things you can do to help prevent bad fuel breakdowns when on the road:

  • Fit a secondary fuel filter/water trap.
  • Take a few spare fuel filters with you on your trip so if you suspect contaminated fuel you can at least install a new filter, which might make all the difference.
  • Use your long range tanks to carry extra fuel that you know isn’t contaminated.
  • Use a fuel treatment to prevent any algae growing.
  • Drain the diesel pump to remove any water that might have made its way through the filter.

Check out the full feature in issue #85 February 2015 of Camper Trailer Australia magazine. 


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