Ok well I just drove the car with the air on full heat and on 1st with high rpms for a minute. It burped and I refilled it. Ran it again and the expansion tank stayed full. The gauge still reads 40c when I start it and after 10mins of driving, it only hit 50c.

I have no idea why the antifreeze overflowed. Possibly I left the cap loose. Possibly overheated. Arg...

No I did not measure resistance of the sensor. Should I remove the new one from the car and test it?

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1984 300td euro turbo