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I don't know of any issues from a T-Stat sitting on the shelf, but if it's older than 4 years, it merits replacing (just to be on the safe side). The date stamp is usually on the bottom of the thermostat, facing into the engine on the lowest disc below the spring.
The original OEM supplier to Volvo when our cars were built was Vernat/Caloristat and they still make a great thermostat (mail order from FCP Groton for $9). I've installed a lot of them in my friends cars, and only had one that was a bit sticky to start with (but it works fine now after a few days of use). All of the others have worked great from day one.
The current thermostat supplier to Volvo is Wahler, and they make terrible thermostats. The Brass metal they use in the thermostat housing is way to soft, and tends to fatigue and break. Most often, it will break one of the two spring tensioned legs at the bottom, and cause the thermostat to open wide and stay open (causing a very long warmup). However, on the rare case they the Brass top fails, it will lock up tight in the closed position and cause an immediate overheat. I've had one friend who had a 3 year old Wahler fail in the closed position, and before she noticed the temp gauge it was too late. It had risen above the overheat marks and the engine head had warped (causing the head gasket to loose compression). She took the car to a shop where she had to have the head resurfaced/planed to restore compression.
If anybody who works for Wahler is reading this post, -I've got a bucket full of broken/failed Wahler thermostats that I've taken out of my friends cars (from tuneups when they bought them). Compare this with one of my friends bought an '89 Turbo 740 that still had the original factory installed Caloristat, which still worked fine.
God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 268k miles.
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