Remove 2-piece,top engine cover for a better view & put a pressure tester on coolant system.
Z has coolant lines that travel from one side to the other in rear,,,they run across from throttle body to throttle body & you can see where they connect into the main coolant log.
I’m thinking water pump is underneath L/H vvel up front.
I’d start with about 10psi & start looking,,,but don’t exceed what’s on cap.
Radiator cap is on plastic tank & flat cap on engine side.
Perform this cold & it should reveal itself.
I don’t advise adding any kinda stop leak, as this will cause future problems.
Good Luck & keep posted.
BTW; list year model of Z
