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.

MB suggests a very elaborate procedure to remove air which I have never tried. The system is designed to purge air naturally if you are patient. Remove wire from sending unit and look at gage. then ground wire and look again. Gage needle should go to extremes. If it does, the sender is probably bad. That's OK, they are cheap. You can still get heat inside even if the t-stat is NG, but it will take longer.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

My 83 SD had the same symptoms. The thermostat failed open.

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Fred Johnson
1977 240D
1983 300SD

1984 300d Turbo 146k
Temp Gage Will Not Indicate, Slow Temp Gage To 80 C, Replace Thermostat, Add Water Top Radiator Hose, Air In Sytem, Works Ok
And Fast Heat