[Badbubba]

Originally Posted by dreksnot

Check valve allows unmetered airflow FROM filtered catch can to the throttle body connection. When positive pressure in the intake manifold (in boost), the check valve closes preventing a "boost leak" out the can. The larger hose connected from the oil filler cap to the catch can serves to vent the crank case, and also allows a slight venting of the valve cover pulling smellies back into the motor under idle and normal driving (no boost).

Not sure why he has two check valves installed (MM includes one in the can fitting and he has the gray one inline).

I have seen oil spattering (as seen in his picture) when my catch can is filling up, needing to be drained.

Yeah. Sort of figured the dirty side was from the oil filler neck, since there was no connection to the valley cover or valve cover. But wasn't sure about the breather. If the breather is open to atmosphere, then that creates a parallel path to the intake manifold side of the throttle. So at low rpm, the crankcase is not getting evacuated very well. That may be the reason Rupert states there is no oil in the can. But at high rpm and increased crankcase pressure, the pressure should vent out the breather without an issue. Imo, the breather should have a check valve to not allow outside air in. Now back to crankcase pressure. If the clean side is connected to the intake tube, for n/a, then that would pressurize the crankcase. Or if the check valves are not installed or working correctly, could cause pressure. Even if the hoses were on the wrong port would cause issues, which doesn't appear to be the case.