could be three things, the common lower intake gasket leaking, or the further down problem of a head gasket. the other problem could be the actual upper intake manifold which should be checked if you are doing a lower intake reseal anyways as there is a class action lawsuit against GM for the premature wear of the plastic, basically the EGR pipe comes up right near the throttle body and it melts away the plastic over time due to the extreme temperatures of the exhaust gas.
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