Bad water pump.
Have you checked the water pump? Did you burp all the air out of the system after changing thermostat? Have you checked for blown head gasket? Sometimes these will leak so minor that it will not have white smoke or water in the oil.
Long time no see,Dandy how you been?
How do you "burp" the air out of the system?
It has been a while. All is good been working on the TOYS. Lol. Burping is different on different models. Some you just keep radiator cap off as car is idling and add water as air bubbles come out. But some cars have bleeder valves built into the cooling system that you open to let the air out.
Looking for a Used Mustang in your area?
CarGurus has 42,625 nationwide Mustang listings starting at $1,699.
Search Ford Mustang Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Mustang Experts