Start Simple
First of all, everyone keeps saying "thermostat gasket"...actually, the Mustang uses a large O-Ring instead of a gasket to seal the thermostat. They are only a couple of bucks at Auto Zone or Oreilly's. Second, I'd replace the spring type hose clamp with a worm gear type hose clamp. As someone pointed out, they weaken over time and should be replaced. Third, depending on how old your car is, while you're at it replacing the O-ring and hose clamp, may as well eliminate immediate future worries by replacing the upper radiator hose while your there.
My 96 GT was doing the same thing not long ago, and I replaced the O-Ring, new thermostat, and still had a leak. Turned out the upper radiator hose was old and appeared fine on the outside, but it was deteriorated on the inside and was actually allowing fluid to seep between the layers of the hose and leak out at the end...At first, I thought I had a cracked intake manifold, and never could get it to leak while I was watching it, but it would leak after I had parked it overnight...I could see the green puddle in the valley between the heads under the intake and couldn't trace the leak...Start simple and replace the easy stuff first...if you continue to have a leak after that, then it may be time to look at the intake itself as the point of failure.
First of all, everyone keeps saying "thermostat gasket"...actually, the Mustang uses a large O-Ring instead of a gasket to seal the thermostat. They are only a couple of bucks at Auto Zone or Oreilly's. Second, I'd replace the spring type hose clamp with a worm gear type hose clamp. As someone pointed out, they weaken over time and should be replaced. Third, depending on how old your car is, while you're at it replacing the O-ring and hose clamp, may as well eliminate immediate future worries by replacing the upper radiator hose while your there.
My 96 GT was doing the same thing not long ago, and I replaced the O-Ring, new thermostat, and still had a leak. Turned out the upper radiator hose was old and appeared fine on the outside, but it was deteriorated on the inside and was actually allowing fluid to seep between the layers of the hose and leak out at the end...At first, I thought I had a cracked intake manifold, and never could get it to leak while I was watching it, but it would leak after I had parked it overnight...I could see the green puddle in the valley between the heads under the intake and couldn't trace the leak...Start simple and replace the easy stuff first...if you continue to have a leak after that, then it may be time to look at the intake itself as the point of failure.