Ive been having some problems after replacing valve cover gaskets (rough idle/ stalling which i think is a vacuum leak, smoke which is either the seafoam being burned by the new plugs or head gasket failure). When I was troubleshooting, i checked for bubbles in the coolant lines. When I popped the radiator cap, I found a small, squarish gasket floating at the surface. Any ideas as to what that might be? It seems like its probably important.

 

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