What is a catch can, and why would you need one?

By Evan Spence 6 Min Read

A good quality oil catch can, and modern common-rail turbo-diesel engines are a match made in heaven. While there are many opinions out there on the internet, from what I have seen with my own eyes, I’m convinced that a (I’ll say it again – good quality) catch can kit is well worth the asking price.

They are relatively cheap, in kit form are easy to install for the home DIYer. They provide peace of mind that harmful carbon-filled sludge isn’t filling your vehicle’s vital components. Such as the intake, EGR and intercooler to name a few.

Internal combustion engines by design create positive crankcase pressure. This adds what is known as blowby into the engine’s crankcase. With modern vehicles, due to complex emission systems, this blowby can’t simply be vented to the atmosphere like in the bad old days. GQ Patrol owners, I’m looking at you…


The modern solution is to feed these harmful blowby gases that contain such friendly byproducts as vaporised oil, NOx, hydrocarbons, carbon monoxide and as mentioned below water condensation, back into your engine to be burnt a second time. 

This sounds awesome in theory. However, as a mechanic mate once eloquently told me, it’s mechanically the equivalent of breathing in your own farts. Not ideal for the longevity of your engine. This is why people fit catch cans to modern turbo-diesel engines. They filter and store as much of these oily diesel farts as possible, instead of feeding them back into your engine. 

It’s not just oil

Something else I’ve noticed is the amount of condensation catch cans retain in cooler weather. This is especially noticeable with vehicles that did loads of stop-start driving, not allowing the engine to get hot enough to burn the condensation off.

The contents of the catch can would look absolutely putrid. More like a jar of mayonnaise that had been left in the sun. I’d rather see this gloop (that’s the technical term) captured in a catch can instead of being fed through your engine’s intake system.

So, what does a catch can do? 

As the name suggests, a catch can is a device that essentially acts as a bucket with a filter to collect and retain oil vapours instead of sending them back into your intake. They connect to your vehicle’s crankcase and filter blowby gases which will turn to thick sludge over time. This chokes the intake of your four-wheel drive’s engine. Not pretty stuff. This is why many people recommend fitting a catch can to prevent this issue. 


This is especially true with brand-new vehicles. Fitting a catch can from day one will give your engine the best chance of living a long healthy life. Think of it as preventing artery-blocking cholesterol buildup by eating healthily from an early age.

How can you pick a quality catch can? 

In kit form, you want to see a complete fitting solution including but not limited to solid mounting brackets, quality hose clamps and sturdy oil-resistant hoses that are large enough in diameter to not impede flow and are form-fitting for a factory-looking fit. 

Another extremely important design element of a good catch can is an integrated pressure relief valve. This will open up if excessive crankcase pressure is experienced, or more commonly if the filter becomes blocked. I’d not recommend ever running a catch can that doesn’t have a pressure relief valve. The best players in the industry include them for a reason. 

Another key feature you want to see in a quality catch can, because let’s be honest a water bottle could be used as a catch can – but it will do a terrible job of it – is an internal filter that is easy to inspect, and cost-effective to replace when the time comes.

Do you need to maintain them?

The best way to think of the filter inside a catch can is as a towel. A towel will absorb water, but has a limit. Once the towel has soaked up as much water as possible, it will begin to drain off the towel.

Most manufacturers recommend changing the filter out every 40,000km as a guide. Replacement filters will cost roughly between $80 – $180 depending on the brand. I’d also have your mechanic check the condition of the filter (or check yourself, it’s not a hard job), clean out the catch can and drain the oil from the unit every service to keep on top of things.

Speaking of draining, good catch can kits will come with a clear piece of hose and a drain valve. This is so you can monitor how much oil has entered the catch can, as well as be able to drain the catch can quickly and easily.  

If you are thinking about fitting a catch can to your off-roader, check out some of the offerings from Ryco. They have kits to suit specific vehicles, universal catch cans as well as a huge range of filtration solutions. https://rycofilters.com.au/catch-cans 


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