Poppet valve & components, including valve stem oil seal. The yellow paint is a spring strength marking. A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapour flow into an engine.It consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem.The portion of the hole where the plug meets with it is referred as the 'seat' or ' valve seat '.t consists of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The portion of the hole where the plug meets with it is referred as the 'seat' or ' valve seat '.