I never measured the PSI but with duct tape over openings I would think 5 or less. Just enough to create enough pressure in the crankcase for the smoke to show you the leak. When the engine is running blowby does the same thing, you just can,t see it well with the engine running. Rear corners of the intake, valve covers, and the disributer are generally the culbrit but intake gaskets and bolts are on the list.