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The seals inside the cam cover that go around the spark plug wells are gone....it's a fairly common issue on most DOHC motors I have seen. If they are not there, there is a gap you could stick a fingernail in between the cover and the well.
When you buy a valve cover gasket kit it comes with the o rings that seal the spark plug holes...if its the driver side you will have to remove the hydroboost and master cylinder in order to get it out