I am sure the catch can collects a pretty high %age of contaminants but, obviously not all. After about 30K with my catch can, I have a very slight film of oil on the back side of the throttle body butterfly. It easily wiped off with a lint free cloth. I think it would be a good experiment to see exactly how much the second can would catch although, like suzieque says, more stuff, more poetential problems. If you do it, let us know how it goes.
I believe the only resistance in the system is the PCV Valve. Try blowing through a catch can inlet and see how much comes out the outlet.
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The Arrington document is an interesting, though they don't mention diesel engines.
The dual catch can in the Arrington document are actually for 2 different breather lines on the gas engines. On the CRD, I am only aware of 1 breather line and I suspect that johngreen1234's question was about putting dual catch can on the 1 breather line.
Also, the CRD does not have PCV, it has a CCV, probably for the reasons described in the Arrington document about why PCV "fail" on high boost applications.
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i'm not sure what a ccv is, but you are correct about PCV valves not being ideal for a turbo engine. when the intake is pressurized by the turbo it creates more pressure than what is in the crankcase, which closes the pcv valve (hopefully) if it doesn't you are just boosting directly into the crankcase. regardless, at the same time air is being pressurized in the crankcase, thanks to blow by at the rings. which causes several problems. i.e. oil blowing out of the dipstick or elsewhere, the pressure can actually restrict oil from returning to the crankcase.
i'm familiar with a few solution. venting directly out of the crankcase is preferred over the valve cover, with either a venturi tube in the exhaust, or a pulley driven pump.
the benefits of crankcase evac pumps are not only true for turbo engines though. it creates a vacuum(preferably 10-14"), having your crank spin in a vacuum is more efficient. i know they would benefit any size motor but i've heard small block chevy's can gain 20hp, some big blocks around 30hp. they have oil/vapor separators & a catch can. and the constant relatively high vacuum pressure keeps moisture from building up and contaminating the oil. i've heard great things about mileage too, but don't have much daily driving experience of my own
i would experiment on my ride with an electric smog pump hooked up to the pcv system to remove all the moisture thats creating the sludge you guys are seeing, but i don't even get the slightest bit of oil in my intake and the breather hose is always spotless clean. pvc valve is original and works great.
i don't think that two catch cans is really the answer though...if one can't catch it all between regular cleanings, then IMO the best option is: removing the breather from the intake system, and evacuating the the system with a different vacuum source.