The thermostatic valve starts closed which only allows the the water to circulate around the engine.
As the engine temperature increases, the water temperature also increases.
Eventually, the thermostatic valve opens which routes some of the water to the front of the car
and through the radiator. This cools the water 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.
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 that might be in the way such as the top engine breather pipe.
Using a ratchet, an extension bar and an E12 socket, remove the 2 bolts holding the valve to the engine.
The following photo shows the placement of the E12 bolts. Normally, these will not be so visible.
Now that the thermostatic valve is free, you can move it slightly which gives you more room to remove the
existing pipe clamps. These are 1 use items so they will be damaged when removed. Placing a screwdriver
into the "ears" of the clamp and twist. Be very careful not to damage the water pipes during this step.
You can now lift the old thermostatic valve out from the engine bay. Remove the temperature sensor as this
will need to be reused in the new valve. Clean the engine in that area before fitting the new valve.
Fit the new valve into position, fit new hose clamps on all 3 hoses and push the hoses fully onto the thermostat.
Tighten all of the new hose clamps fully to 5Nm. These normally require a 7mm socket.
I use Jubilee clips and the sizes required are:
2x 30mm to 40mm
1x 22mm to 30mm
(You may need smaller if your pipes are more compressed).
The outside diameters of the valve outlets are (from left to right) 30mm, 19mm, 27mm.
Now insert the bolts and tighten them down to 10Nm. Refit the coolant temperature sensor.
Reattach any pipes or items that you previously disconnected or moved to gain access to the valve.
If you lowered or tipped the engine, put it back before refilling the coolant and bleeding the airlock.
Thanks to Peter G for most of the photographs.