There is a part that sits on top of the overflow/coolant reservoir on the 1995 Blazer 4.3 engine that I am missing. I have no idea what it is called and so I'm having trouble replacing it. Does anybody know what it is? Thank you

 

That's a terrible picture, so it's pretty hard to see. But it looks like the PCM, which I don't believe is supposed to be in that location on a 95. It is on the later ones, though.

It wouldn't even start or run without a PCM, so I doubt yours is missing.

 

To what I see on Google, but not yours, is that big black ball(vacuum reservoir)?

Bit a better piccouldhelp

 

FWIW, this is my 2000.