There does seem to be a lot of confusion on pretty much any forum in regards to catch cans and what they do.
First I'll start off with what a catch can is;
-A container that catches fumes and vapors from the valve assembly or crank case prior to being introduced back into the intake system for emmissions purposes.
Now this is a bit different than a breather or breather style catch can;
Breather - a filter that replaces a valve case or crank case tube that usually recirculates the fumes to the intake for emmissions purposes, with the filter, fumes and vapors are now vented to atmosphere.
Breather style catch can - A container that catches fumes and vapors from the valve assembly or crank case and vents through a filter to atmosphere rather than back into the intake system for emmissions purposes.
Now that we know what our options are, why should we and what is our best option?
Why should we install a catch can?
The reason to install a catch can is simple, to catch those fumes and vapors to prevent them from re-entering the intake system. These fumes and vapors could be anything from oil, to fuel, to condensation (yes.. WATER!).
These fumes reduce performance, longevity and can actually reduce the octane level in your cylinder during combustion that steals fuel economy and could cause harmful detonation. Grime and sludge also build up on the valves and cylinder head further making a mess of your engine.
Which option then is best for you?
Well it honestly depends on your engine and to what degree you plan on adding protection to your vehicle, also local and state/provincial laws regarding emmissions control.
A simple catch can is usually the safest route to go because it covers all your bases. It also allows you to keep vacuum on the system which helps to collect the vapors.
Just a breather filter on isn't always the best or legal method as those vapors will go straight to atmosphere. Usually the filter gets all plugged up and could leak caught vapors down the side of your engine or get sprayed under hard acceleration. This method also doesn't allow you to use your engines vacuum to help remove those pesky fumes.
A breather filter catch can helps keep your engine bay clean by trapping the fumes but also allows the valve assembly or crank case to breathe however it likes to. Again, though, it doesn't allow you to use the engines vacuum to aid the process.
Anything I should know for installing one?
Make sure though that a catch can NEVER sees boost from a turbo vehicle (such as our 2.0T's), boost going into a catch can can just end up pushing all that sludge in one gunky mess right into your intake and cause massive issues.
PCV valves are another consideration, they are probably the #1 contributer to gunk and condensation, they should have a catch can installed on them, again though, make sure on turbo applications that the can isn't routed to see boost.
NEVER install a catch can on an external EGR system, it could cause a fire or damage your engine.
Ok, it's installed... now what?
Well just like your air filter or tires, it needs to be checked and serviced regularly. Check it every oil change (5000km's or 3000miles) or every time you open your hood. For the first 5000km's try and check it every 1000km's, that way you'll be able to tell how much collected is normal. It's perfectly normal for your car to burn through a litre of oil per service interval, expect around 60 to 100ml (2-3oz) to end up in the can.
But you'll have more than just oil in there, there will be fuel that escaped while the valves were open, condensation from valve cover and crank case during hot shut downs. How much of the 'other' stuff will depend on anything from how rich your car is running, altitude, how hard you drive and even environmental things such as whether it's been raining a lot lately.
Once you know what's normal, start looking out for things that are not normal. If your catch can came with a magnetic plug make sure it's always clean, you should never have metal shavings in your catch can.. if you do.. it's a sign you've got some serious issues and your car should NOT be driven until it's been inspected.
Coolant is another thing that should NEVER been in a catch can, it means you've got a seal or gasket that's broken and leaking coolant into your engine. Again, if it's there... don't drive your car.. you'll just make things worse.
To check for coolant the easiest method is to use a glass container to collect your catch can contents.. then freeze it in your freezer. Water will be at the bottom with lots of heavy sludge, then oil, then coolant, then fuel. The best way to check is the freeze method as it causes everything to seperate and become easily identifiable.
Any pictures for me to reference against?
What's a good outline without pictures?
Here is a freshly caught catch can sample after slightly over 5000km's.
See how its slightly seperated at this point but we still can't say for sure how much is oil and how much is condensation or anything else.. but there is a lot of sludge in the bottom... ew.
After being stuck in the freezer over night the oil is clearly visable in contrast to the sludge and water. There is a very thin layer of fuel on the top.
Empyting your catch can:
I'm just adding this for a complete guide, but your catch can contents are considered oil and need to be disposed of correctly in accordance with all federal, state/province and local bylaws and regulations. Even though there may be water or fuel in it it should be accepted by any place that disposes of oil after oil changes, the environment levy was paid when the oil went into your engine so no additional expenses should be expected. Not to sound like a hippy but think of our environment and be responsible.
How to Install