So, I assume many of you know the difference between a good and bad thermostat, but for reference, I've created a picture to show the difference between a functional thermostat and a bad one.

The test to check the thermostat is simple. Just put the thermostat you're doubting in a pot of water and boil. Depending on the kind you have, it'll open at or around operating temperature, which will vary from vehicle to vehicle (160-180*).

The one that was in my car is on the right, and the left is one that came out of a 240sx. Their function is the same regardless of size.


At 200*, the thermostat on the left is fully open.



Notice how the spring on the left thermostat is mostly, if not fully compressed in contrast to the right.