Spray the old grommet with PB Blaster a few times over a few days; it should soften it up a bit, then get the longest pair of vice grips or pair of pliers you can find, then either use a hammer handle as mentioned above, or do what I did; use the engine lift hook for the fulcrum point and lift twist lift twist lift twist, etc until it comes out. The old grommet will crumble so have a pair of long needle nose pliers ready to pull the pieces out that fall onto the little shelf under the grommet inside the valve cover. Of course, don't do this until you have a new grommet in hand.
