My 87 300E did that because the sensor in the expansion tank went bad. I suspect your sensor has failed. The fact that you just replaced the radiator is either a coincidence, or you disturbed the sensor enough in the process to send it over the edge.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)