can blow by cause a vacuum leak condition

[prljeep]

I have the blow by problem that is so common on many 87-90 I6's.

The other day I was checking the vacuum tank under the front bumper, due to a lack of 4wd. I pulled one of the hoses off and covered it with my thumb, which cause the engine to smooth out and drop idle speed. So, can the usual blow by condition actually plug up a part of the vacuum system causing the vehicle to act as if it had a vacuum leak?

Thanks in advance!
(90, MJ, I6)

[Bender]

If your rings are severly worn they can pressurize the crank case enough to actually force the flow of crankcase fumes into the intake. This can give the appearance of a vacuum leak.

[Matthew Currie]

I would guess it's the other way around. If, for example, you have a hole in the vacuum tank or another vac. leak, you'll have low vacuum (of course), and that will prevent the crankcase vent from working, adding to the blowby problem. Of course it's typical for these to have a blowby issue anyway, but good vacuum is essential.