Thermostat stuck open ? Not necessarily...
I wasn't getting heat from out of the vents and my back up temperature gauge( I also have the one in the speedo cluster), that reads degrees, was only going to 160. I also had a leaking heater core. So I thought I have to replace my thermostat because it's stuck open. I found out that if there is air in the system then the sending unit may have an air bubble or just air around it and the heater core may have air in it as well, and so the sending unit wouldn't get hot and therfore would give me a low reading on the temperature gauge and the heater core wouldn't get hot enough for heat from the vents . I also learned that what the temperature rating on the thermostat is pretty much where the engine stays ,once open, as long as it's working correctly. The temperature on the temperature gauge depends on where you have the thermostat. Don't get me wrong don't think your engine is still okay if your temperature gauge says the temperature is high. Just wanted to share
1984 jeep cj7, 258 I6, rebuilt T176,rebuilt Dana 300, amc 20 , dana 30, 4.10's Trac-lok in rear , 4.10's detroit soft locker in dana 30, 2.5 inch bds suspension lift, 1 inch polyurethan body mount lift. Line ex. weber carburetor. factory tach. factory clock ,oem replacement speedomter cluster, and oil pressure gauge, factory volt gauge.