I have an old Ford Taurus I am going to let my son drive to school and back. The car has not been driven for about 3-months so I decided to take it out for drive. I went about 25 miles and when I got home and shut it off I heard a bubbling sound. I popped the hood and the radiator cap was extremely hot and the coolant reservoir was so hot it was bubbling. While I was driving the tempature gauge was normal, however, I did notice when I went to start it back up the temp gauge did read towards hot, but when started it it went back to normal. Any ideas why the reservoir was bubbling?
If the reservoir is bubbling, the coolant has gotten WAY too hot and is boiling. The temperature gauge might read normal if the temperature sender isn’t working as well as it should.
I’d guess the thermostat is stuck shut. Change it out and see if this stops.
Change the radiator cap, first. A bad one (ten years old, right?) will let the coolant, normally about 220F degrees, boil out.
The thermostat OPENS at 190F degrees, but, the coolant temperature safely goes, and stays, at 220F degrees if the radiator cap (and, the rest of the cooling system) holds.
If you have one of those stick-on temperature indicators, you could place it on the radiator cap for the next run. Then, you could see if it’s getting TOO hot.
Agree with hellokit that this should be done first. If that does not resolve the problem, suspect a failed water pump next, and finally, a leaking head gasket with combustion gasses working their way into the cooling system. This will invove a major repair to repace the head gasket.