Thursday I drove over three mountains, and every time I was going down, the coolant temp started to drop and after a long period of coasting, engine braking, and light to zero load the needle would go all the way to the bottom. Then as it flattened out the temp gauge would normalize.

Any ideas?

I bought a new thermostat which I'm gonna fit when I do my timing belt but could it be something else?

I should say that under normal driving conditions everything is normal.

Perhaps air bubbles rising to the top pipe where the coolant temp sensor is?

(​A guess)

Thermostat could be stuck open. I read somewhere if you open the radiator cap WHEN COLD and start your engine and there is no movement of water the thermostat is ok but if the water is moving about then its stuck open.

Even bad connection on the sensor will do this and is more likey that than the thermostat if it is going to zero, engine movement will effect it be it on power or off power and the tilt of the car, check the single spade connector and the spade that is rivet to the sensor as these can work loose.

 

Jeff

Mine used to do this, when I cornered too fast or accelerated heavily it would drop to zero. The connector on front of the engine was breaking contact.