I flushed coolant today, had beensittin for a bit before I bought it and I know the coolant will bad but anyway here is my question. Replaced thermostat andfilled the coolant. Now system is full but gauge is still erratic. Gauge come up just below 105 but drops below 40 while driving but with that said the fan turns on even though gauge is reading below 40. Car is or doesnt seem to be running rich when gauge is registering cold so I'm leaning to believe it's a sensor for the gauge problem? Anyone have this issue or input on my problem?

 

Replace the Temp. sensor located in the drivers side cly. head. What kind of trmp gage do you have (celsius)???