81 300 SD temp indicator

[rscott77x]

Recently, my 300 SDs temp needle does not seem to get much above the first line on the gauge. I had a new temp sensor and installed it but no change. I am thinking I may have an air bubble so the sensor is not immersed in coolant. However, I do have interior heat. Could a failed thermostat cause this? That is, is the default failed for the thermostat wide open? I have the original BEHR radiator (very scary). Could that be a factor? I plan to change the thermostat but would like to know the best way to get the air out of the system after doing so.
I am still in love with this car after 24 years.