What is fuel contamination and what should you do about it?

Five minutes after pulling out from a servo you notice your car doesn’t sound quite right. You go to accelerate, and there’s not the power there should be. A warning light flashes up on the dash. The odd sound gets worse.

It might be fuel contamination, and in this article we’re going to look at what that is, what you can do to prevent it, and what can be done if it happens to you.

Your engine runs on a mixture of petrol or diesel and air, and anything else will have a detrimental effect on the engine’s performance. This is why there are filters to the air intake and the fuel system, and our focus today is the fuel system. Fuel impurities can be split into two categories: matter and foreign fluids.

Matter is tiny particles of things like flakes of rust, dust, leaf debris and the like.  Foreign fluid is typically water, which is as good for an engine as ingesting petrol is for you.

Contaminated fuel can mean result in needing to replace the fuel pump, lines, injectors… you’d be lucky to get away with a $5,000 bill and nearer $10k is perhaps more likely.

The RACV told us they have attended 1345 cases of fuel contamination over two years, so let’s say that’s 700 a year out of around a million callouts, so about 0.07 per cent of callouts. Around 750 of these were in regional Victoria, out of about 200,000 regional callouts (about 0.2 per cent). The figures are quite low. 

But why are you more likely to encounter fuel contamination in rural areas? One reason is because regional servos do not cycle through their fuel as quickly, giving time for contamination such as condensation to form, leading to water ingress and even ‘diesel bug’, which is a bacteria that enjoys growing at the junction of diesel and a tiny amount of water. It’s a double whammy – a foreign-fluid and a matter-contamination problem. Modern cars with high-pressure, finely tuned limits are less tolerant of poor fuel than yesterday’s cars.

The warning signs

Here’s how you can tell your fuel is contaminated. Shortly after leaving the servo, say 5-20km, you’ll notice signs such as:

• engine running rough
• the check engine light comes on
• difficulty starting
• lack of power

You must stop immediately. If you suspect fuel contamination, you are unlikely to be able to perform a roadside diagnosis, but should contact the servo and inform them. Many contaminants, water included, sink to the bottom of the tank where the pickup is, so waiting for everything to settle then draining a couple of litres may help.

Your insurance may cover you for fuel contamination. You can also report the incident to the federal Department of Climate Change, Energy, the Environment and Water, and to Consumer Affairs in your relevant state.

How to avoid fuel contamination

There are a few steps you can take to reduce the risk of contamination:

Don’t fill up from empty 

In some cases, contaminated fuel can be diluted. For example, if you accidentally put a litre of petrol into a diesel tank that’s three-quarters full, you’ll probably not do any damage. But put 70 litres of petrol into a diesel tank that’s on the red needle and you’d best not even turn the ignition on and will need to call a misfuelling service quick smart. This is where a long-range tank can be handy; just keep on topping it up, right? However, this method doesn’t offer complete protection as fuel is taken from the bottom of the tank, which is exactly where water and many other contaminants will sink to, so in many cases adding new fuel won’t help.

Don’t be the first to fuel up

Bad fuel can either be delivered, or it can go bad when it is stored. If you’re the first car through after the tanker has visited that increases your risk.

Avoid high-risk fills

For example, a decrepit looking servo that’s poorly maintained and has a fuel delivery schedule measured in weeks. Any recently flooded area is suspect too.

Visual check

Light aircraft pilots learn to drain a sample of fuel from each tank and visually check it for colour, water and presence of matter. This is not a foolproof method as an inspection may not catch all bad fuel, but sometimes you will be able to see problems. To check, decant a glassful of fuel into a transparent container and inspect. You’ll need to be familiar with what good fuel looks like first though, so practice at home with some fuel, water and fine dirt.

Keep the receipt

This doesn’t prevent any problems, but it does help your argument.

Prevention is cheaper than cure

Now for the money spending part. Your carmaker has already fitted a fuel filter which deals nicely with the matter type contaminants down to maybe 2-5 microns. So why not fit an extra filter down to 1 micro? Because of how modern cars monitor fuel filters.

Your car’s computers will be looking at the fuel pressure before and after the filter and calculating the difference. When that difference builds up to a certain amount, the car decides the fuel filter is clogged and needs replacing. If you add another filter, and also one which is even finer than the original, then you’re making it harder for the fuel to flow and that might trigger a fuel filter clogged warning. If the carmaker also decided that, say, 2 microns was okay… how far do you go? One micron?

An alternative method is to add a larger-grained filter earlier in the system, say 30 microns, somewhere between where the fuel nozzle goes in and the tank. You may ask why bother, when the OEM filter will catch, say, 3 microns. The answer is to reduce the load on the OEM filter. However, fuel filters are relatively easy to replace and cheap, so this idea is of limited value.

Now for foreign fluid, specifically, water. This is the biggest risk because of immediate damage to the engine, rust, and bacteria. Some carmakers do include a water trap, and even if one is included, some people fit a replacement or secondary trap. There are chemical treatments to kill the bacteria, but those treatments just mean the dead bacteria simply clog up your fuel tank, and don’t remove the water in which they lived. If you are going to fit a filter, I’d go for a quality water trap.

You will also hear of pre and post filters. A pre filter is one that the fuel gets to before the factory filter and is coarser than the factory unit. A post filter is one after the factory filter and therefore finer.

Here is an example of a second fuel filter from Diesel Care which will remove water and trap particles down to 5 microns in size:

Remember that risks like fuel contamination cannot ever be entirely eliminated even with the most careful fuelling and all the filters in the world. All you can do is understand the issue and decide how much effort you want to expend to avoid a problem that is unlikely to occur, but costly in time and money when it does.

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