The Heater core on the inside of my 65 mustang on the passenger side starts to leak when it gets to operating temperature. It is spilling all over the inside of my car. Does anyone know how i can fix it?

 

The first thing you want to do is to inspect the hoses where they connect up to the core. If it is not leaking there, then you will need to replace the core itself.

 

Also, Glen, does my memory serve correct in thinking that the original heater plenums were cardboard or something? I remember replacing mine and the core as well and I was thinking this was something that was fairly inexpensive and easy. Also, as Glen said, check the connection. They are bad to leak there.

 

the cardboard phelum has nothing to do with leaking coolent. that is just for defrost to work. i replaced mine, and they are plastic now.

 

Yeah, whenever I replaced my heater core, the plenum just fell apart in my hands. The aftermarket plastic ones are far superior to the old cardboard ones.