I have an 85 740 turbo automatic. When the car is warming up, the temp gauge is moving up towards the middle as is expected, but passes the middle slightly (somewhere between 1/2 and 3/4), and then falling (assuming the thermostat opens) to about 1/3. Is this normal? The temp then starts to climb again to 1/2 and the cycle continues over and over, however it seems to cycle more slowly. After the car should be fully warmed up (after 10-15 minutes of driving and more), the temp gauge is still moving between 1/3 and 1/2 but never seems to settle on 1/2. This cycling and the fact that the temp gauge never settles on 1/2, seems abnormal to me. I did replace the thermostat about a year and a half ago with a Volvo original thermostat. I have had some overheating problems before and after the thermostat was replaced, but finally I figured out that the fan clutch was not working properly and was the culprit to the overheating, so the fan clutch was replaced. The car never boiled over, but was just running too warm at around 3/4. Since then the overheating has stopped, but now the car is overcooling. My plan is to replace the thermostat again, since the overheating may have damaged the thermostat (just to eliminate the possibility that the thermostat is the problem). My question is if there is anything else that may cause the problems I'm seeing. Other than the thermostat is there anything that can cause overcooling? What about the heater? I know there is a vacuum operated heater valve on the coolant hose going to the heater. If this valve is stuck open, will this cause overcooling (heater acting as a second radiator)? If this is the case how can I check that the valve is working properly? What about the temp gauge or the temp sensor? How can I verify that these are working properly? The temp gauge is not moving erratically, so I don’t think this is the problem, but you never know. Is there anything I have forgotten, that I need to look at?

Any suggestions for how to solve my car’s cooling problem are appreciated. Thanks.