I am trying to bring an old 89 xr4 150hp (S/N 0C184436) back to life following a voltage regulator fire nearly a decade ago. Wasn't sure if the fire was caused by poor circulation to the regulator so I figured I would go ahead and replace all components of the cooling system which includes the poppet valve.
So I pull the poppet valve cover off and someone has definitely replaced it in the past, but also behind the smaller side of the cover there is a plastic plunger piece that has an o-ring on it which looks like it's used to block water flow completely to that port back into the block, and I can't figure out if this piece is supposed to be in there or not. I've checked parts diagrams for the motor and I can't find this piece anywhere on there so I was hoping someone can identify it and let me know if it should be put back in or not.
I've included a picture of the piece I'm talking about.