Mazda Miata 1990-1997
You can test a suspect thermostat with simple kitchen tools. First, remove the thermostat from the engine. Next, boil 3 quarts of water in a large pot. Tie a string to the bridge of the thermostat and suspend it in the boiling water, in which you-ve already placed a candy thermometer. Watch the thermometer and thermostat. Note the temperature at which the thermostat starts to open and that at which it is fully opened. It should be fully opened at the rated temperature, plus or minus 5 degrees. If not, replace it.