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Do you need a secondary fuel filter for your 4WD?

Modern diesel engines are extremely sensitive when it comes to fuel. By that, I mean that if you put poor quality fuel through a modern diesel motor, you can do some extremely expensive damage. The problem is that in Australia, our fuel quality is surprisingly bad compared to many other parts of the world, and that’s a recipe for disaster.

One of the more common preventative measures is to install a secondary fuel filter for your engine, to assist with providing clean, quality fuel. The thing is though, do you really need one, or is it just another marketing ploy?

Types of secondary fuel filters

There are two places that these secondary fuel filters are mounted; before the factory fuel filter, known as a primary or pre fuel filter, or after the factory fuel filter, known as a secondary or post fuel filter.

In both cases, the diesel comes from your fuel tank and goes through the factory fuel filter, but you are installing another fuel filter before, or after the original one to assist in the goal of sending clean fuel to the engine.

There are benefits to both installations, with the general consensus being that you always have a finer filter after a coarse one. Fuel filters filtering abilities are measured in microns, with the smaller the number being the finer the filtration.

For example, you might have a filter fitted before the factory one that filters down to 30 micron (or 30/1000ths of a millimetre) and then through the factory filter that is 5 micron (5/1000ths of a millimetre).

30 micron pre filter

Our 30 micron pre filter, in our Dmax

Pre or post filter?

There are two trains of thoughts here. The first is to fit a filter prior to the factory one that does most of the work, and keeps your factory filter in good shape, and gets rid of any water that comes through.

This is usually a 10 – 30 micron filter placed prior to the factory one. If you are going down this path, make sure it has a clear bowl and a water trap to stop any water getting through, as this is one of the biggest issues with Australian fuel.

Pre filter kits are very popular on new vehicles, as some manufacturers will not cover damage done to motors with filters fitted after the factory one. This means if you want some extra protection without voiding your warranty you do it prior to the factory filter, or not at all.

The second train of thought is that the factory filter isn’t actually up to the job, and your fuel should be filtered to a lower micron level, in which case you install a finer filter after the factory one.

The Isuzu Dmax factory filter is 5 micron, and a lot of people with common rail diesels fit 2 micron filters after. This will likely upset the vehicle manufacturer in the case of a warranty claim, but it does guarantee the fuel going in is very clean.

Provent 200 install

Our 30 micron pre filter can be plumbed as a post filter if you prefer, running a smaller micron element

Watch the flow rate

One thing to be mindful of when installing aftermarket filters is that you don’t restrict the flow rate too much. The finer the filter, the harder it is for fluid to pass through, and you can make the fuel pumps work very hard trying to filter your fuel.

There is a video on YouTube showing diesel flowing through the factory filter, then a two micron filter, and then another 2 micron filter before going into the engine, with the pressure gauges and flow rates shown, which shows there is no problem. However, what happens after 10,000km, or 20,000km when the filters are full of gunk?

Many 4WD’s have fuel lights that will come on if there is a restriction, and knowing where the sensor is becomes important to identifying what filter is blocked, and knowing when you might be doing damage to your fuel pump.

On the Dmax, if the factory fuel filter is starting to block, the light will come on when you give it a bootful, and usually at higher RPM, and then it will go off when you are just cruising. This is the sensor saying I’m having a hard time getting fuel through, please change the filter. Its not an immediate need thing, but you should be doing it as soon as possible.

Change your filters regularly

No matter what setup you have, the one thing that works well is to follow the manufacturers instructions for replacing fuel filters, or do it more often than what they recommend.

Be aware though, that every time you open your fuel filter (or whoever is servicing the vehicle does) there is a chance that some gunk will get in, and you do more harm than good. In essence then, change the filters regularly, but don’t go overkill on them.

Maintenance parts

Good quality preventative maintenance will make or break your 4WD

Water alarms

The last, and common option today is not to bother with a secondary filter, and just to fit a water alarm. These go prior to the factory fuel filters, and make a loud noise as soon as they detect water, giving you time to shut the motor down and do something about it.

Ideally you have them set up so they filter any water out too, and there’s plenty of people just running a water alarm and trap, and no additional filtration.

Draining water from fuel

Most factory fuel filters have the ability to drain water; read your manual!

Mr funnel

An interesting design that I came across some time ago is called Mr Funnel, and it is exactly that. When you fill up with fuel, you send it through the funnel, and any water particles are collected and trapped prior to going into your tank. This is a great idea.

Fuel additives

You can buy a number of fuel additives that supposedly emulsify the water into diesel so it can ‘safely’ be run through your motor. There are also additives that help to clean the system and do a whole heap of other magic. Whether they work is not something I’m qualified to talk about, but we always carry a bit of additive with us, if needed.

Algae growth

Diesels are notorious for getting algae growing in their tanks. Once its in there, there’s not a whole lot you can do about it, asides from treat it with a fuel additive.

Algae is most likely to grow in tanks which are not used regularly, and not kept full. In our 80 series, the vehicle was not driven on a hugely regular basis, and I ended up with a nasty algae bloom that eventually blocked my fuel filter, causing issues getting fuel on acceleration.

The original owner had installed a tiny fuel filter, which worked well in stopping things going through the motor (even though it was largely unnecessary being a 1HDT which would run on almost anything), and we ended up in a car park at Jurien Bay replacing a fuel filter (lucky I had a spare!).

Do you really need a secondary fuel filter?

I can already hear people chanting ‘you don’t need a secondary fuel filter’, and they certainly have a point. There’s hundreds of thousands of vehicles in Australia that only have their OEM filter, and are doing just fine. 

However, if you are travelling out of the bigger cities and towns you are more likely to put a bad batch of fuel in, and that’s when having a secondary fuel filter can help. Every few weeks there’s a post online from someone who has done major engine damage because of fuel that was contaminated, or full of water.

Post fuel filter

A 30 micron pre fuel filter that we run in our Dmax

If you shut your engine down fast enough you can get away without any damage (you’ll get a warning come up showing water in your fuel), but its not always possible, or easy to do this.

For us, we’ve gone for a 30 micron pre filter as it doesn’t affect the warranty, the fuel flow sensors still all work correctly and it will do a good job of filtering water out, and sending it to the bottom of the bowl. I’ve seen specks of crud in the bottom of the filter, but nothing to worry me too much. We try to fill up at reputable places, and having a long range fuel tank has helped that dramatically.

Long range fuel tank

Our Long Range Fuel Tank means we skip the smaller fuel stations that turn over less fuel and have more chance of contamination

Do you need a secondary fuel filter? No. Is it a good idea? Probably, but it depends on where your fuel comes from, and the level of risk you are willing to take. If you lived out in the bush and had a higher chance of bad fuel I’d say an absolute yes. If you never leave the city its less important, and regular filter changes can make a world of difference.

Ultimately, pick your poison. They aren’t a must, but they are certainly a good preventative measure, and we’ll always have one on our vehicles.

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