How long did it take for either event to happen after you started the engine? It sounds like a rapid pressure build up. About the only thing that can cause that would be head gaskets.
Run a pressure test on the cooling system and see if it holds pressure. Then with the gauge still on, start the engine and watch the gauge. You should not see a rapid pressure increase. If you do combustion gas is getting into the cooling system.
It should not be related to the valve covers.
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Thursday, February 15th, 2018 AT 9:18 PM