In addition to what Paul said, check the orifices in any fittings that
screw into the manifold. Some of them have a small hole that can get plugged,
and if it does, your PCV is NF. Also if you have a PCV valve in any of the
hoses, clean or replace it, being sure that it goes in the right direction
(air should flow toward the manifold but not away from it).
Check and replace any hoses that are brittle, cracked or don't fit tightly.
With the orifice with the small hole, I recommend drilling the small hole out
to 1/16" or 1.5mm to get a little better flow and less clogging.
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George Downs Bartlesville, Heart of the USA!