Catch Cans


Oil Catch Cans and Diesel Filtration

By Nathan Johnson | 26 April 2021

A common add on to most newer 4WDs is an Oil Catch Can and a Secondary Fuel Filter. If you are sitting on the fence regarding fitting up either of these features, please read on.

Oil Catch Can

Let us start with why we would install a oil catch can. One of the first emission controls to become popular on all petrol and diesel engines was a system called Positive Crankcase Ventilation (PCV). This allows the positive pressure created in the engine’s sump to make its way out via a PCV valve usually mounted in the valve cover of the engine. This positive pressure released via the PCV valve has oil mixed with it and requires the engine to keep this oil contained, so it does not end up on the pavement whilst driving or parked. The PCV valve is connected to the air intake piping/manifold so the engine can burn this vapour off and out the exhaust it goes. It sounds straightforward, and yes it is.

Now let’s enter into the 20th century, and we are now governed by a worldwide set of emission control rules for our motor vehicles set out by a governing body that’s role is to lead us to zero or close to zero emissions.

Let me introduce to you the Exhaust Gas Recirculation System (EGR). The EGR is exactly what the description says it does. You may be wondering why we would want to recirculate our exhaust gasses? This is to further reduce noxious gasses from our engines. It has been said that if we recirculate the exhaust back into the combustion cycle, it will reduce those noxious gasses, therefore, reducing our carbon footprint. In a nutshell, the exhaust is sent back to the intake manifold, and at certain operating conditions, the EGR valve will let the engine eat its own exhaust.

Now mix these two together, (PCV and EGR), and what we end up with in most diesel engines is a very thick paste that blocks up your manifold, restricting airflow and impeding your vehicle’s performance.

So what does the oil catch can do to help? It takes away the oil that is the by-product of the PCV and sends it through a filter allowing the vaper and pressure to return to the intake. This means that the EGR can operate as intended, and the PCV is now being filtered. The catch can is then drained at your service intervals.

The Secondary Fuel Filter

Doesn’t my vehicle already have a fuel filter? Why would I need a second filter?

To put it bluntly, Australia has inferior diesel fuel quality. I do not believe that we make any diesel anymore on the east coast of Australia, and therefore the majority of it is imported vie vessels from overseas. This type of fuel handling, in my opinion, is very loose and would introduce many contaminants into our vehicles.

The standard filter on your 4WD is very good; most are filtering at 5-7 microns, even the aftermarket filters we often use during scheduled servicing are the same to meet OEM specifications. They do have a sediment function, and most have a warning light to advise you of either sediment or blockage. What they don’t do very well is separate out moisture. Moisture in the diesel fuel system is not ideal, and the diesel acts as a lubricant. A secondary filter of good quality will do that very well.

So do I fit a Pre or Post Filter? 

A diesel guru once told me that the pre-filter is a better option than the post, but they both have significant benefits, water separation. A pre filter will typically be 30 microns of filtration, and this allows for great flow to the OEM filter, and the 30 micron filter will catch all the big items and let what will pass go through to the OEM filter, knowing that it has a finer filtration rate of 5-7 microns. The pre-filter has also separated a lot of the moisture, and now the OEM filter only has to filter the fuel and not worry about moisture.

A post filter would have a similar setup though the filter would be 5 microns instead of 30. The same guru as above said to me, ‘why would you want to filter your fuel twice at the same level. The only benefit would be the water separation’. I see his point, but if you are using a filter that is not 5 microns in the OEM housing, then this set up is excellent as a second defence.

In my opinion, the pre-filter is my go-to option.

  • It separates the water early, allowing the OEM filter to just worry about fuel, and if any moisture does make it through, it will be trapped by the OEM filter.
  • Allows better flow to the OEM filter.
  • Catches the big contaminates.

There is always plenty of discussion around pre and post-filtration, and there are benefits to both of course. If you are having these discussions and have one or the other installed, you are already winning.

Johnson’s 4WD supply and install the Direction Plus range of Oil Catch Cans and Secondary Filter products. To discuss your options, please contact us via our socials, our website, on the phone or drop into the workshop.