I just got my car back ('96 850T) after having it towed to the shop on Monday morning - I was in stop and go traffic on the freeway when I notice some steam coming from the front passenger side. My temperature gauge read "HOT! HOT! HOT!" so I pulled over to see that the coolant was boiling over and out from the overflow reservoir.

I had her towed to a local reputable shop (Bob Workman's Volvo) and after pressure testing the system found no leaks but did find the following:

1) thermostat was missing..... no not malfunctioning, but missing as in not there at all.

2) the fan motor was dead.

I've had this car for about 4 months and this is the first time this has happened. So apparently, I was running without a thermostat without any real issues. It probably was when the fan motor failed that it all went down and I overheated.

Just thought I'd share in case it might shed some insight for anyone.