I just bought my first Mustang recently and decided to change out all the fluids regardless of what the previous owner said he done and noticed that the coolant reservoir was empty. I had been driving it around for some time and the engine temp. never even got close to overheating. So I waited for the engine to cool and then I filled it with coolant to the "cold fill" line. Later that day I drove to a city about an hour away and as I was walking away from the parked car I noticed that coolant was leaking everywhere all over the ground. So I thought that the thermostat was stuck and promptly bought a replacement. All was well for about a month, then I got home one afternoon and it was leaking all over once again. It overflows until there is nothing left in the reservoir, even when the engine's cool. Is this new thermostat stuck? or is there something more to this?

thanks for the help