If Zaid's excellent How can I test my thermostat? is "Thermostats 101" then this might be the beginning of "Thermostats 201"…
My VW Jetta TDI (diesel) has been running cold (about one or two marks on the temperature gauge) instead of being pegged to at 190° – courtesy of the ECU, the gauge doesn't really read temperature anything between roughly 170° F and 210° F is display as 190°. The heater output has also been feeling low and the engine has been taking longer to warm up.
All in all this seemed to me like it was a thermostat problem (as opposed to a temperature sensor problem) since the heater wasn't putting out as much heat as I expected and also because a sensor problem usually goes with a somewhat erratic temperature gauge and mine was nice and stable, just low.
So, I got a new thermostat from my friendly (really) local NAPA store and put it in last night. After a test drive the problem seemed resolved – gauge is back to 190° and I've got plenty of heat. But I couldn't leave well enough alone and I put the old thermostat in a pan of water with a thermometer and started heating it up. The spec on the thermostat is for it to open at 195° and that's exactly what it did. Took it off the stove and as the temperature dropped past 190° the thermostat was well on its way to closed, by 160° it was closed completely.
Now I'm puzzled. Seeing this happen makes me think that the old thermostat might not be bad. Or, is it taking too long to close? So I went for a drive, I needed a haircut anyway, the new thermostat really does appear to be working better.
So, what I'd like to understand is:
What should the hysteresis curve of a good thermostat look like? If it starts to open at say, 195° on a rising temperature, should it be finishing closing at the same point as the temperature falls?
Are there failure modes besides "stuck open" and "stuck closed?"