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Steve, if the black plastic disk is something like this, then it is the PCV valve, which controls the flow of gases from the crankcase of the motor. If you pull it off you will find that there is a valve inside it, and you can blow through it one way, and can't suck back the opposite way. Helps stop the buildup of exhaust gases in the engine.
This one is off my GU 2.8 so is probably similar if not the same as yours.
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