For the amount of money it will cost it would be worth changing the thermostat anyway. They aren't very expensive and reasonably easy to repair.
Water pump replacements are more expensive AND more work. Try the cheaper one first as thermostats are known to wear out.
If the overflow tank is backing up it will be because the safety valve built into the water pump is diverting built up pressure.
SOURCE: Saab 9-3 turbo problem!
Sounds like the recirculation valve (aka diverter valve) is failing. A lot of turbo cars that have them end up with failures from them, since they're usually cheaply made from plastic, and the diaphragms inside don't hold up over time.
What the valve does is, when you shift, there is a backsurge of boost air that hits the closed throttle plate. The diverter valve (DV) opens and dumps the air back into the intake piping. It's kept closed during acceleration by spring and manifold pressure. When you shift, the pressure drops on the backside of the valve and it opens, venting the excess boost air. If your valve is failing, just the pressure of boost in the intake piping could be pushing it open, causing the acceleration to slow. I'd look into a replacement diverter/recirculation valve.
SOURCE: Saab 9-5 Aero Wagon 2000 wont restart when warm or after running
I have a 1999 Saab 9-5. I had a similiar problem. The car would always run cold. It would get hot and the engine would quit. It could be anywhere. One mile from start to 50 miles from start. Once it cooled down it would re-start but the cool down time was always different. Only difference for me was my car would crank, just not run. Replaced the crank position sensor ($60), no more problems! Good luck!
SOURCE: engine overheating
the problem is your water pump some have a plastic impeller that brakes up which stop the water serculating around the engine thats what causes the engine to overheat. when you replace it make suer the impeller is alloy and change the thermastat has well just to be on the safe side. if its blowing out gray smoke the head gasket needs replacing as well
SOURCE: My 92 Saab 900 blows cold air constantly and the
Unless you have symptoms of over-heating, I rather doubt that your water pump is a problem. I recall that my water pump had a metal impeller (1992 C900); I haven't heard of a plastic impeller on a Saab pump before. (Most pump failures are the bearing seal, and the pump leaks coolant) If your temp gauge is slightly low (and was OK before) I'd rule out water pump issues.
You might have one or more of the following issues:
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