Mod Details
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What Are The Symptoms Of A Failed Thermostatic Valve?
The thermostatic valve starts closed which only allows the the coolant to circulate around the engine. As the engine temperature increases, the coolant temperature also increases. Eventually, the thermostatic valve opens which routes some of the coolant to the front of the car and through the radiator. This cools the coolant down to regulate the temperature of the engine.

The valve is designed to fail in the fully open position. What you will notice is that the engine doesn’t get up to full operating temperature and you’ll have almost no heating in the cabin. The other, less likely, alternative is that the valve sticks closed and the engine has no way to cool itself. This normally means full temperature in 5 minutes followed by a warped engine head.
Changing The Thermostatic Valve
The valve sits on the top of the engine just behind the fuel pressure regulator. Actually getting to it is tricky so you might want to lower the engine to make more room.

On top of the thermostatic valve is the coolant temperature sensor. Disconnect the electrical connection. Disconnect or move any other pipes/hoses that might be in the way such as the top engine breather hose.
Credit
Thanks to Peter G for most of the photographs.
Thanks to Tim for the information about the Click-R pliers.