Take out your old thermostat, test it with a pot of water that you set on the stove. Use an instant-read thermometer to monitor the water temp as it goes up (keep the probe off the sides of the pot!) and check for certain when the thermostat starts to open and when it is fully-open. That way you will know what it is doing.

Packman

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!