A youtube here, how to test a car/truck thermostat to see if it is opening properly and not stuck. It is the same principle no matter what car or engine you have. https://www.youtube.com/watch?v=aa1yG-fntRk
I test even a NEW thermostat before I install it. Better check before you install, otherwise additional work. A thermostat is a wear item and breaks after years of operation in the area where it bends to move the valve. So better change it in certain intervals depending on operating hours or years of operation.
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Failure Modes
Mechanical thermostats fail in one of two ways:
1.Stuck-closed
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.
2.Stuck-open
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.
Test Procedure
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:
•if the thermostat opens, this confirms the thermostat is able to open
•Remove the thermostat from boiling water²:
•if the thermostat returns to the closed position, this confirms the thermostat is able to close
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?
Why engine coolant temperature won't give the full story
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:
•faulty engine coolant temperature sensor
•environmental conditions
•radiator fans not turning on (too hot)
•radiator fans running when they shouldn't be (too cold)
•blockage in the coolant piping (too hot)
•compromised water pump (too hot)
•leaks (too hot)
source: mechanics dot stackexchange dot com
Shogun tricks and tips for the E32 series are HERE!
It's also useful to know that the temperature stamped on the device is the temperature that it will begin to open. It may not be fully open until 10F or so higher.
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