You might as well change out the thermostat when you flush the coolant on the SAAB 9-5. Both are easy jobs. It's a little messy, but satisfying when you are finished.
Symptoms of a thermostat stuck in the open position includes a lack of heat inside the car and the temperature gauge on the instrument panel never reaching up to the 9 o'clock position. Symptoms of a thermostat stuck in the closed position includes the temperature gauge reading extremely hot and possibly a repair bill for thousands of dollars sitting in your seat when you pick the car up from the dealership after it stops running due to overheating.
I encourage you to pay the few extra dollars for a thermostat that is build to lock in the open position when it fails.
I purchased the MotoRad fail-safe thermostat from my local Autozone, the price was $6.
You will need a pair of pliers, a pan to catch the coolant, two gallons of undiluted coolant, at least four gallons of distilled water, a running garden hose (if you are the adventurous type and your tap water isn't too terribly hard otherwise buy some extra distilled water and skip the garden hose part), 13mm and 10mm sockets with matching extender and a socket wrench.
The system holds -7.4 liters- of coolant/water total. You will want a mix of between 50/50 and 75/25 coolant to water when you are finished. You won't have to over think it, the car is built to make this mixing automatic, the reservoir and radiator will drain half of the liquid while the other half stays in the system. I'll explain this part in detail when the step comes.
Start by losing the protective plastic panel from under the front bumper, like you would for an oil change. Slowly unscrew the lid from the coolant reservoir. Let any pressure release before completely removing the cap.
On the driver's left side of the engine bay, near the headlight, at the bottom of the radiator is the the green plastic petcock used to release the coolant. Place the catch pan under this green petcock to catch the coolant.
Reach down from above and turn the green petcock counter-clockwise a quarter turn and then pull it out to release the coolant. Pliers can be gently used if it won't turn by hand. The part is made of plastic, so care should be taken not to tear it apart. You can push the petcock back in at any time to stop the flow of coolant.
Let all the coolant drain out while you move on to the thermostat part of the project.
The thermostat is on the back corner of the engine. There is a large coolant hose that goes from the radiator to the thermostat housing.
The hose can be seen in the bottom right corner of this image.
Follow the hose to the thermostat housing and use pliers to release the clamp and pull the hose free. Some coolant will spill out.
The thermostat housing is attached by two 10mm and two 13mm bolts. Remove all four of them. The 10mm bolts hold a ground wire and a bracket, the 13mm bolts clamp the house to the engine. The space is tight so use an extender and take your time.
After the four bolts are removed, pull the house off and take out the old thermostat.
The rubber washer that comes with the thermostat has a valley on the inside so it can be placed around the metal disk as shown.
Stick the new thermostat back into place, taking care to point the little hole in the disk towards the top. The disk may also be labeled to show which direction is the top.
Bolt the housing and ground wire back on and securely reattach the hose. The coolant will be drained by this point.
Temporary close the petcock and fill the system up with water. The correct method is to use only distilled water in the system, but I used the garden hose to flush the system because I like to tempt fate. Use your own judgment, tap water can plug up your system with minerals.
Wear gloves while working with the hot engine and coolant to prevent burning yourself.
Once the system is filled with some water, and with the reservoir still open, start the car, turn on the heater inside the car, and give it a few moments to warm up so everything starts flowing. Now is the time to check for leaks on your thermostat job.
Once warm, open the petcock to start releasing fluid while adding fresh water to the reservoir to keep the system full. Do this until the liquid from the petcock runs clear. (if you are like me and are using a garden hose, switch to adding your four gallons of distilled water.) Close the petcock, shut the car off, and allow everything to cool down. You now have water in your entire coolant system. Let the car stand until cold.
Here is the secret to mixing 50/50 coolant. The car, when cold, holds onto half of fluid and release half of fluid. Open the petcock one more time and it will release half of the water in the system. Close the petcock and lock it with a quarter clockwise turn. Add your undiluted coolant until the reservoir reads full and you now have a 50/50 mixture in your system. Isn't that neat.
The water and coolant will mix quickly as the engine runs. Once the coolant is given time to mix and any air is released, top off the reservoir with more undiluted coolant.
A day later you will want to check the coolant with a "coolant tester". These can be the simple eyedropper like devices that use specific gravity to check the coolant mixture or strips of paper treated with chemicals that offer more detailed results. My eyedropper type tester was $1 from Walmart.