How can I tell whether a mechanical thermostat works as it should?
Is there a test that can definitively tell me if a thermostat is good or bad?
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Mechanical thermostats fail in one of two ways:
The pintle/spring doesn't retract to permit flow at the set-point temperature that is usually stamped on the thermostat itself. which can lead to the engine running at hotter than expected.
If hot enough, this can lead to serious issues such as a compromised head gasket or warped cylinder head.
The pintle/spring doesn't actuate to maximize flow restriction at temperatures cooler than the set-point temperature. This usually results in the engine running at cooler than designed-for temperatures.
Running an engine too cool can result in issues with the emissions and catalytic converter efficiency due to increased hydrocarbons in the exhaust.
The thermostat needs to be out of the car and on the bench for this.
The temperature at which the thermostat opens is usually stamped on the thermostat itself. In the absence of such, a technical manual or parts supplier will require referencing.
Appropriate PPE is strongly recommended as this test involves boiling hot water.
Place the thermostat in boiling water¹:
Remove the thermostat from boiling water²:
¹ - remember that the temperature of boiling water varies with altitude; it's 100 °C at sea level
² - now what did I say about PPE again?
Just because your engine temperature gauge isn't where it should be doesn't mean that the thermostat is bad.
Here is a list of other plausible reasons that could cause the engine temperature gauge to read too cool or too hot:
Just ran into a variation of the stuck open failure. Thermostat opened and closed at the correct temperature (195 F) when tested in a pot of hot water. However, when installed, the thermostat would allow water to pass at temperatures 40 F cooler. Replacing the thermostat fixed the problem.
Edit for dlu: I can read the coolant temperature on the vehicle info center. With the old Tstat the upper hose would get warm at about 150 F indicating coolant flow. It didn't get much higher while driving in 30 F weather. After checking the old Tstat operation while out of the car, I put it back in with the same results. The new Tstat holds 192 F.