where is the fuel pressure regulator at on a 1999 4.2 v6 and also by checking it, do you do it while its running?

 

I don't know on a 4.2 but I assume it's on the fuel rail where the fuel line comes in . You need to remove the vaccume hose and see if gas leaks out if it does it's no good. Then put a vaccume guage on and start it up. It should read like 5-10 psi.