It’s an unfortunate fact that contaminants can get into our vehicle’s fuel supply. It can be from a little rust inside the tank, or a bit of grit in the servo’s holding tanks, or even some bulldust caked around your filler that falls in upon opening the cap and is then washed into the system by the fuel delivery. It’s almost inevitable, really.
This first unfortunate fact is compounded by a second one: modern common rail diesel and multi-point petrol injection systems cannot handle grit in the fuel. We’re talking several thousands of dollars’ worth of damage if a high-pressure fuel pump cops a few grains of sand through it, scoring up a seal or two.
Of course, this is generally avoided thanks to your fuel filter, which is there to clean your fuel of contaminants before they can damage more expensive stuff down the line. The issue is though, long-distance driving, particularly of the off-road or remote country variety, can introduce more opportunities for contaminants to get into your fuel and wreak havoc.
This is why most folks fit an auxiliary fuel filter.
They’re an extra layer of defense, and almost negate any potential fuel system problems – cheap peace of mind when you’re out in the middle of nowhere.
What’s a micron or two between mates?
Fuel filter efficacy is determined by the micron rating – the smaller the micron number, the more the fuel is filtered. A single micron is equal to one thousandth of a millimetre, so we’re talking microscopic levels here. For reference, a human hair averages about 70 microns thick.
Aftermarket auxiliary filters are broadly split into two categories, pre-filters (primary fuel filters) and secondary (final) fuel filters. The decision on which one to buy really comes down to your personal preference, but we’ll explain both below.
Primary fuel filter (pre-filter )
A primary filtration kit is installed between the fuel tank and the factory fuel filter. The Diesel Care product in the previous link is rated at 30 microns (factory filters generally are around 10 micron) and is designed to catch all of the larger particles that may be sitting at the bottom of your fuel tank. It’s also designed as a water trap, so if there’s any water that’s managed to get into the fuel, this will act as a first line of defense before it hits your factory filter, or worse, high-pressure pump.
Diesel Care recommend the primary filter is changed at the same time as the factory filter (unless there’s water ingress, at which point change it immediately) as part of routine servicing.
Secondary filter
A secondary filtration kit allows fuel to pass through the factory filter before it runs through the final filter on its way to the high-pressure pump. The Diesel Care final filter is rated between 2-5 microns and is there to make sure that pretty much nothing larger than a politician’s idea of integrity can get through. These also function as a water trap, however, if you do see water in the glass bowl then it means it’s already passed through your factory filter as well, so you’ll have to change that out too.
These don’t have to be changed as regularly as a pre-filter or factory filter as they’re filtering already-filtered fuel, so they last longer. Once every second factory filter change is the go here.
Our recommendation
Look, fitting either a primary or final fuel filter should be looked at as cheap insurance for your fuel system. There are countless nightmare stories about people being stranded hundreds of kays from the next human being and looking down the barrel of a very expensive tow followed by an even more expensive replacement of their fuel pump and injectors as well as a full overhaul of the rest of the injection system, which is about as much fun as sharing a public bathroom stall with the Incredible Hulk.
Regardless, either one will go a long way towards safeguarding such an eventuality (the fuel system thing, it’ll have no bearing on the Hulk thing) and is often one of the first modifications made to a touring rig.
Your fuel can’t be too clean
If you’re really going to hold a knife up to our belly and demand an answer, we’d probably go with the secondary filter. You really can’t “over filter” fuel and the reassurance knowing that if anything does manage to get through or damage your factory filter, the final filter is still in place to catch it.
Also, it’s worth noting that there’s no 100% foolproof way to prevent contaminants from getting through. We spoke to Diesel Care for this article and some of the stories they told us of damage caused from under-filtration made us pre-emptively check our credit rating with the bank. They did stress, however, that the best and most effective way to prevent melting your credit card and ruining your trip on account of dirty juice is to filter it, then filter it again.
Your fuel can’t be too clean.
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