Leaking Intake Manifold Gasket (found in the "Engine" category) - Vacuum Gauge readings below 17-22 in-Hg (normal manifold vacuum at sea level) at idle may indicate an intake manifold gasket leak.
Note: As elevation increases, readings decrease. Subtract 1 in-Hg from normal manifold vacuum reference for every 1,000 feet increase in elevation above 2,000 feet.
Stuck open PCV Valve - remove the PCV Valve and gently shake it; if it does not rattle, the valve is clogged or dirty and should be replaced.
Cracked Intake Boot (found in the "Engine" category)