Why your 4X4 needs a secondary fuel filter

By Wes Whitworth 10 Min Read

With modern common-rail diesel engines having such fine tolerances, a secondary fuel filter is a must to ensure a batch of dodgy fuel doesn’t cause havoc. 

Wander around on Facebook, or lean over the fence and ask your neighbour about fuel in Australia, and you’ll find out our stuff is little better than crap. Car makers complain about it too, saying we can’t get their latest and greatest vehicles here because of the quality of our fuel… but, there’s a but.

See, the quality of diesel in Australia is considered to be amongst the best in the world. Literally. According to Strata Industries which is one of the agencies responsible for ranking fuel quality around the world, Australia, in 2019, was rated as having the 11th best diesel in the world. Indeed, we sit alongside countries like Sweden, Germany, Japan, Finland, Denmark, the Netherlands and Portugal with Australia and these countries having diesel that contains between 10-15ppm (parts per million) of sulphur.


But that doesn’t mean we don’t have problems in this country. Because we do. See, while the quality of our diesel is good, the fact that service stations in this country are so spread out and demand can fluctuate can mean that quality (because the diesel has gone stale) can cause problems in modern diesel engines with their fine tolerances. And that’s why secondary fuel filters are such a necessity.

There are more than a few secondary fuel filters on the market these days, especially so with the proliferation of common-rail diesel engines. Tolerances within the fuel injection system have become so minute, that the smallest debris or contamination can cause injector damage or malfunction and potentially destroy an engine.

In this article, we’ll look at what a secondary filter kit does, the different types of filter kits available, the different sizes of filters and how installing a secondary kit can stave off disaster.

The factory fuel filter

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The standard factory filter I have on the old 80 Series. We’ve all got one, but is it up to the job? 

All four-wheel drives have a fuel filter in them. They’re usually under the bonnet, have a water trap to catch water, a sensor to detect any water, and should get replaced at every other service.


Insofar as their filtering ability is concerned, an OEM filter is usually around the 10 to 12-micron mark (0.01 to 0.012mm – or 0.0004 inches for those who think better in imperial) – but not exclusively greater than 10; sometimes as small as 5-microns. So they will filter out exceptionally small particulates and debris. Where the worries begin is that particles as small as 2 – 5-microns diameter can cause issues in the fuel system of newer common-rail diesel engines.

So what aftermarket options are available to help ensure we’re getting the cleanest fuel into the engine as we can?

Pre-vs-post filter

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A Flashlube pre-filter. Diesel runs from the tank, to the pre-filter, then into the factory filter, before ending up at the injector pump.

There are two options for additional filters on the market – pre- and post- (final) filters. As they sound, one is installed before your factory filter (pre-filter) and one after (post/final-filter). There are good arguments which way to go, whether pre- or post-filters, which we will discuss further in to the article.

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You’ll see the aftermarket pre-filter on the left side of the engine bay, with lines that run over to the factory filter on the right, via the firewall.

Generally, the pre-filter, will be somewhere in the vicinity of a 30-40 micron filter sizing, and is designed to capture any larger contamination before it gets to your factory filter. Pre-filters also include a water trap, to catch any water that’s been brought up from your fuel tank. Being the larger size, they catch a lot of the larger particles and will keep your factory fuel filter cleaner, longer.

The post-filter has a much finer filter and will stop particles of 2 – 5-micron getting into your fuel system, plus include another water trap as well. Most vehicle manufacturers state that anything below their factory filter size (below 5 – 10-microns as an example), will not upset the fuel system – injectors, pump, engine.


There has been quite a bit of debate as to which option is the better, and we’ll explore the reasoning here.

The better option?

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This is the bigger particulate matter the pre-filter stops from running into your factory filter.

Debate as to which is the better option has gone on almost as long as there have been diesel engines… We already know the differences so why would you install a pre-filter over a post-filter, or vice versa?

On the one hand, a pre-filter picks up the bigger contaminants, and has a glass bowl water trap, which traps water, and can be visually checked. This will keep your OEM filter cleaner, longer, and should stop a lot of the water getting to your OEM filter sooner; by the time the factory light / buzzer goes off for the water trap having water in it.

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The clear bowl on the bottom of the filter traps water, and lets you drain the bowl should you get a load of water in your fuel.

On the other hand a post-filter picks up the smaller contaminants and also has a water trap. Your OEM filter still gets the larger particles, and will clog up sooner with this, however only particulate matter smaller than 2-microns will get through the filter.

Final thoughts

Don’t get me wrong, I own a dirty old 80 Series LandCruiser mechanical diesel, where you can just about lob a tin of Heinz soup into the fuel tank and it’ll keep running without an issue.

However, for common-rail diesels, in the past I have run a post-filter as opposed to a pre-filter for a couple of reasons. First and foremost, I change my fuel filters religiously; every second service (10,000km). So keeping my filters clean and replacing them is on me, not on a pre-filter. And second, with the inbuilt water trap, by the time my ‘water indicator and buzzer’ go off, I’ve still got the water bowl in the post-filter to fill up. Which should give me time to pull over, shut the engine down and drain the bowl or find out how much water I’ve got in my tank. Having a pre-filter without an inbuilt buzzer, by the time the factory alarm goes off, water will be quickly getting into the fuel system and breaking things.

Vehicle specific

Something else worth thinking about is vehicle manufacturers’ ‘within specification’ responses. Most of us would recall the Toyota issues with dust getting past the factory air-filter. There’s been more than a few D4-D 2.8-litre HiLuxes go in to limp mode as the Mass Air Flow sensors have been covered in dust. There has also been a few that have had “dusted engines” – where dust has caused scoring on internal components and caused premature engine failure. This is all despite Toyota unequivocally stating that the dust getting past the factory air-filter was too small to cause damage.

Add this same manufacturer logic to “anything smaller than 10 – 12-micron particulate matter will not harm your fuel system”. It’s not that I don’t trust the manufacturers, but I’d rather be doubly safe, especially due to how much we invest into our vehicles.

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