To check for a vacuum leak on the under side of the intake manifold, remove the PCV valve from the valve cover and seal the valve cover opening.
Remove the cap from the other valve cover and start the engine.
With the engine running, place a business card over the opening in the valve cover (where the cap was removed).
If the business card is held in place by a vacuum, then you have a vacuum leak from either an intake manifold gasket or a cracked intake manifold.
John
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