I have a vent hose coming off the cam cover, running about 5 feet back to an air/oil separator then another hose goes back to the intake tube between the MAF and the turbo. Could this be drawing in enough unmetered air from the crankcase to cause random lean conditions and stumbling?
If it's plumbed right, the air should be metered air. Into the engine downstream of the MAF and back out somewhere in the manifold- so that normal operation (where the manifold is under a depression for most cars) will naturally flow air through the crank.
If it's not plumbed that way, then, yes, it can.
If you have air leaks into the crankcase that air will be unmetered. A smoke test is really the only good way to discover these. You can also rig up a slack-tube manometer (easily made at home for little money) to see what sort of vacuum you've got in the crankcase at idle.
maybe im not understand your set up right but how i've ran catch cans in the past is a fresh air line from intake tube before the throttle body and in this case after the MAF to valve cover/crankcase, then have a line from valve cover/crankcase to catch can then a line from catch can to intake manifold. That way your intake manifold vacuum is drawing fresh, metered air into and through the crankcase which will then pull through through the catch can.
Green line is fresh air pre-TB and post-MAF. Blue line is from crankcase, through catch can, and to manifold vacuum.
I have a turbocharged application, I would have to run a check valve or something to prevent boosting the catch can. It's a race car so I'm just going to vent to atmosphere and see if my stumble is gone.
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