Thermostat replacement?

The thermostat is located between the water outlet and a housing built into the intake manifold.

Disconnect the negative battery cable.
Remove the air inlet duct.
Drain the cooling system to a level below the thermostat.
Remove the thermostat outlet-to-engine retainers (usually either 2 bolts or 1 bolt and 1 stud), then remove outlet from the intake manifold.
Remove the thermostat from the housing, noting the orientation for installation purposes.

To install:

Carefully clean the all traces of the old gasket or sealer from the housing and outlet.
Install the thermostat to the housing, oriented as noted during removal, then position a new gasket (if used).
Place a 1 / 8 in. bead of RTV sealant in the groove on the water outlet sealing surface, then install the outlet while the sealer is still wet.
Install the outlet retainers and tighten to 21 ft. lbs. (28 Nm) for the 4.3L 19-96 models or to 14 ft. lbs. (19 Nm) for the 1997-99 models.
Properly fill the engine cooling system and check for leaks.
Install the air inlet duct.

this guide will help walk you through the steps of changing the thermostat with diagrams below to show you how on your car.

https://www.2carpros.com/articles/replace-thermostat

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

Follow the upper radiator hose to the engine / thermostat housing. Under the housing is where the t-stat is located. Drain the coolant below the thermostat level, remove the upper hose, remove the retaining bolts on housing, and remove the housing. Pay close attention to the positioning of the old t stat. Make sure to install the new one facing the same direction. Also, make sure gasketed surfaces are clean and dry and reverse the process.

Those Blazers used Dexcool from the factory and it had a tendency to plug up the heater core. Try having it backflushed (just the heater core) to see if it will work. I have seen them so plugged up they had to be changed

Yes you need to change the thermostat, that is a very very common issue on those vehicles, I do atleast one a month. The p0446 signifies that you have a leak in you evaporative system and could be anything from a bad or loose fuel cap, to a broken hose to a faulty valve. You need to take it to a shop that has the specialed equipment and knowledge to diagnose that code.

Your air bound. Run vehicle with the cap off until it warms up and all of the coolant will get sucked low in the radiator then fill the radiator itself and you will bea all set. There are no other problems just air bound. With the cap off and it running it might bubble out of radiator until the thermostat opens and then go empty real quick. This is normal, wait until you see it go empty then fill it. If your uneasy about this method then fill the radiator and let it get hot like it is now then let it cool and then refill radiator and repeat process until all the air is out. You cant fill a empty radiator from the overflow, it just wont work. Let me know how you make out

Hello,

Not to step on any toes here, but to ad to mastertechtim. May I suggest jacking up the front of the vehicle as high as you can Safely Do. As everyone knows Air will Rise to the Highest Point. So if there is air in the engine and you jack it up it will work it's way toward the front of the engine were it will be easier to expel.

These engines bleed out pretty quickly. This wont be a on going problem, just one more time adding coolant should do it. This system has a heater core bypass that goes to the top of the radiator and pushes the air through the system pretty fast. Keep us posted.

First off, you can't flush a radiator once it gets crusted up inside so if the radiator is the problem, a new one is the only thing that will resolve that.
Since I'm sure you have already overheated it pretty good recently, there is a real good chance of a blown head gasket at this point. Your first step is to pressure test the cooling system and resolve any pressure loss issues or leaks found.

With so much air in system it does not build up enough vacuum when its low to pull the coolant in. When its full it only takes a little vacuum pressure to pull it in. For example when pressurizing a coolant system thats half empty it takes alot of air volume to fill in the void but when its full, only a little pressure is needed. Anyways happy motoring

You need to check to see if the temp is actually getting that hot. Rent a thermometer for testing radiator temp. With the engine cold, open the radiator and insert the thermometer. Allow the engine to warm up. The coolant should range between 195 and 205 degrees. If it goes above that, chances are the radiaror is plugged or there is other engine trouble.
Let me know what you find.
Joe