Basically the placement of the turbo, the size and path of the drain line, and pressure from the feed line all play a part. This is what I think is happening on mine: My feed line is too large, so I placed a restrictor in it, but it still has too much oil-pressure flow to the turbo. The oil going into te turbo is pressure fed, but gravity drained. Oil draining from the turbo for some reason can't drain fast enough (Stock TII turbo is pretty low, and the fitting on the front housing is higher than stock). So the oil backs-up in the turbo causing the bearings to flood, and oil to come out the oil seals. Adding vacuum to the crank case makes it pressure fed, and vacuum assisted gravity drained. So with vacuum it drains adequately and doesn't smoke.

The reason it smoke if sitting at idle for awhile has to do with where I have the crank case vented; in the turbo inlet. That means the vacuum will be proportional to the air flow, so at idle the vacuum is insufficent to help drain the turbo. If you were to hook it up to the TB you'd fix the idle smoke, but when you hit boost, you're actually putting pressure in the crankcase, so it will smoke when you've been on boost for a while, or just afterwards.

The solution for both situations would be to put a vacuum diaphram switch (I think the engines all came with one) on one of the crankcase vents, attached to the TB for both the switching actuator, and the vent. And on the other vent, piping it to the turbo inlet. That way you have descent vacuum in the crankcase at idle from the TB, and vacuum from the turbo inlet when on boost. You'd also have the added benifit of actually moving some air through the crankcase from turbo inlet to the TB at idle, clearing away all the gas and water faster than with just one part vented.

Also, what oil feed line size are you runing? whats your oil pressure? Have you tried a restrictor in the feed line?

 

Your sounds like my setup. My oil feed line is the stock size, its pretty small, but I have a oil restricer plate mounted where it goes into the turbo. The size of the restricter is about 5/64" , the drain is 3/4" My oil pressure at idle is like 25 or so. But Im idling a little high. The oil pressure at 4k is like 80 psi.

 

heres what I was thinking i was going to do.

the fitting on the filler neck is supposed to be the air in, and the the one on the center plate is supposed to be the outlet (I think, from the emmissions diagrams I have seen. Bit If im wrong, this still works.

using a fitting/nipple that has vacuum/boost, put a restrictor and a check vavle on there to only bleed boost and not affect vacuum. This gives you a small volume of air (which can be varied by various restrictors)

connect this to the oil filler neck.

from the lower fitting on the center housing, to a oil catch can and then vent the other end.

should work great.