There are many systems at play in the modern diesel engine, and a few of these are focused on reducing emissions.
Most modern diesel engines utilise Exhaust Gas Recirculation (EGR) or a Diesel Particulate Filter (DPF) to reduce emissions such as nitrous oxide and soot in the exhaust.
This, together with new diesel engine technologies like common rail, can choke your engine intake. Accessories like an oil mist filter or catch can are designed to help combat some of the issues.
What is an Oil Catch Can?
On modern 4wds, primarily turbo-diesels models, small quantities of gas from the combustion process in the engine trickles past the piston rings and into the sump.
Crankcase ventilation stops this pressure from growing by releasing it on older 4wds straight into the atmosphere, whilst on newer 4x4 vehicles it is recirculated into the air intake to be burned in the cylinders.
This air is heavily laden with oil mist, precisely the kind of thing that we don't need hovering in the air we inhale, nor in the air our engines want to breathe.
A catch can's main job is to separate the oil from the air so the oil can be stored and either disposed of periodically or fed back into the sump where it can continue to lubricate your engine.
They are a passive device that won't alter your engine's fuel economy or performance directly and isn't an illegal modification.
Instead, a catch can will reduce maintenance costs and minimise any potential issues.
Why would you need one for your 4wd?
Late model turbo-diesel engines are filled with an array of sensors, valves, filters and pipes required to keep them running safely and efficiently.
These engines typically vent the crankcase gases into the air intake, just after the air filter.
This feeds through the turbo and intercooler via the intake piping before entering the engine.
The turbo won't be much affected by a small amount of extra oil; the temperature a turbo runs at will keep the oil vaporised.
The intercooler is attempting to get the heat out of the intake air, and as temperature decreases, the oil mist creates droplets and sticks to the intercooler's internal surfaces.
It won't take long for a heavy breathing diesel engine to partially block the intercooler and inlet manifold sufficiently to provoke a loss in performance.
Another obstacle that will occur is with the Mass Air Flow (MAF) sensor, which regulates the fuel injection via the ECU.
A coating of dirty old oil doesn't allow the MAF sensor to read the airflow precisely, leading to incorrect fuelling and economy reduction.
If the MAF sensor is entirely out of its expected range, the ECU will implement limp-home mode, significantly reducing vehicle driveability.
Finding a suitable catch can to match the engine size is essential
What makes a great catch can
As we mentioned, a catch can separates microscopic oil droplets from the crankcase air before the air is sent back to the engine via intake and the turbo.
And of course, if a can is 0% effective, it means that it won't remove any oil from the air, while 100% effective removes all oil from the intake air.
Another consideration is the pressure drop. Air must be vented efficiently.
We can measure the change in pressure from the inlet to the outlet of the catch can.
Ideally, pressure on both sides of the catch can is even, meaning the air is crossing through without restriction, rather than building backpressure attempting to leak out upstream of the catch can and thus not flowing correctly.
The ideal catch can will have a 100% oil capture efficiency and zero pressure drop.
The Inlet Manifold Problem of Modern Diesel Engines
Most of today's diesel engines are turbocharged for better performance thus possibly creating more blow-by gases in the crankcase that escape via the PCV system.
This oily mist is redirected back into the inlet and can stick to the inside of the hoses, manifold and turbo turbine blades as well as the intercoolers.
When soot-laden exhaust gas is introduced into the manifold as part of the EGR system, it can create a sticky mess that can dramatically restrict your manifold flow.
This build-up of soot and oil, together with the engine's heat, can bake on to the inlet manifold and eventually reduce the internal diameter of the inlet manifold.
Reducing Oil in the Inlet System
We recommend an oil mist separator's fitment or what is more commonly called a catch can to combat the build-up of oil from the crankcase ventilation system.
The catch can is a filter that sits between the outlet of the crankcase ventilation system, where it comes out from the valve cover, and the PCV valve where the mist is injected back into the air intake piping.
The catch can separates the oil from the air, and returns the oil-free air to the engine. The oil is then collected in a hose, a reservoir or plumbed back into the engine sump.
This reduces the oil in the intake system and leads to a cleaner engine burn and intake system.
Fitting a Catch Can to your Diesel Engine
Spelab provide catch can kits. The kits are vehicle specific. A general kit is also available to fit vehicles that do not have a specific kit available.
We would recommend having your inlet manifold professionally cleaned regularly to ensure it is free of oily build-up and soot from the EGR system.