I put a small catch can on my 99.5 Jetta TDI months ago. I got the car about a year ago with 350k miles on it and no history. The intercooler had a lot of oil in it, so I put a drain(screw) in and check it often. I still get some oil in the intercooler, but it's not at critical state yet. I put a small catch can on it months ago to try to determine if the oil comes more from the top of the engine or the turbo. There is a lot of turbulence under the oil fill cap, but I don't know how much is acceptable, honestly. The catch can I put on also catches some oil, but it is not excessive. I'm thinking the intercooler may never have been drained, but who knows. Now, getting to my question(s)... My catch can is not 'vented', and is a closed system essentially. I see some catch cans with vents/filters on them. Is this better/necessary, or is there not much difference in the two? If I were to just run a vent down under the car and do away with the catch can altogether, would there be issues with the hose being open, or are diesels different? I'm open for catch can opinions and suggestions. Thanks.