1998 Coolant issues at manifold.

Had same issue with My 1998 GT Mustang. Intake manifold has like O-ring type gaskets around every air intake tubes and water line holes. You either will have Original intake manifold or if it were ever changed out, as mine was for the "Dorman" intake manifold. I found my set of O-ring gasket set at Advance Auto parts store in stock. My leak was coming from drivers side rear, water would go into plug hole and make engine Miss.

The work was a do it Yourself with patience. Take photo's of wire and vacuum hose routings, label all connections. So all wires, hoses, and injector wires (injectors can stay in the manifold. If You take injectors out, and they are old and dirty. You will need new O-rings on them to seal). will either lay to the firewall or will lay to the front of the engine. Take photo of wire routing of the Crankshaft Pick-up Sensor. This intake manifold is made of plastic (unbelievable) and the passenger side rear water hose inlet can break. If I remember correctly a whole New intake manifold w/O-ring gaskets $225.