I have a Chevy 2005

The metal round pin In between the bolts one picture has it with it in the other with it out and can be easily inserted back in but will pop back out


V6 4.3 L the other evening I want to start my vehicle it sputtered and I noticed that fuel was shooting up from the area under the air breather cover I remove the cover and saw that what looked like to be a metal pin or plunger had popped out fuel was shooting everywhere I’m thinking this is some sort of pressure relief does anybody have a suggestion it also popped out with a rubber O-ring in a plastic sleeve on it