Just as my wife pulled up to the house after a 30 mile trip from home, Her 91 940 suddenly dropped all of the radiator coolant on the ground. Upon investigating, I found that the plastic nipple on the heater valve had broken off. The remaining plastic was very soft and brittle, and I'm suprised that it lasted as long as it did. Looking for a cause of the rupture, I pulled the thermostat and placed it in boiling water. It wouldn't open and apparently the stem had frozen. I figure that the car suddenly overheated and the excess pressure blew the rotten valve. The purpose of this post is to alert, or remind readers of this potential problem. The plastic that this OEM valve is made of (half of it anyway) deteriorates with age and can lead to sudden loss of coolant. Lukily it happened at home for me, and was a easy fix. On the road would be something else. Hope this helps.
steve