[Vwdiesel] Blow-by test?

[Joe]

I recently saw a vid on You Tube for a different brand of diesel, and they
were checking the engine for a "blow-by" test.  They pulled off the oil fill
cap and would lay a small piece of cardboard over it.  If it doesn't
"percolate" then it's OK?  What does this mean?  And what is an acceptable
amount of "percolation"?  Because my 82 Rabbit does it quite well.

Oh, I guess "percolate" isn't the right word... Was thinking like an old
coffee pot.  Hopefully you know what I mean?

Joe