How to replace the thermostat

need to jack up and remove drivers side tire and using long extensions unbolt the thermostat housing from the lower side of engine or remove a few thing [alternator belt etc] and go through top its tight but the following link has video of both ways of doing replacement
How to replace thermostat 4 2 envoy or trailblazer

hey my friend the thermostat in not like others its a real pain here is exactly what you have to do if u have the 4200 vortec How to replace thermostat 4 2 envoy or trailblazer
btw in case u was wondering or did not know the trailblazer and envoy is the same

Pain- need to remove drivers side tire to get to it. Need to remove alternator bracket too. Top bolt is the hardest to get to. While you\'re in there, replace the temperature sensor too...it\'s right above the thermostat.

The thermostat on this vehicle's In-line 6 cylinder engine is on the driver's side of the engine, below & behind the alternator. You have to remove the alternator to gain access to the thermostat housing. You also need to disconnect the battery before you remove the alternator to avoid sparks and shorts.

Most directions outline removal of the alternator to gain easy access to the hose clamps, bolts and housing of the thermostat. I have located a site that after free registration, has detailed instructions on how to do this job, and the link below that describes an easier method with photos. Check out:

http://forums.trailvoy.com/showthread.php?t=54225&highlight=replace+thermostat

This is a bugger to replace, but can be done with patience. Must remove the alternator and alternator bracket in order to access all bolts on the thermostat.

1. Remove the serpentine belt by using a 3/8" extention to stick in the square hole in the belt tensioner and turn clockwise to loosen and remove belt.
2. Remove the alternator and alternator bracket. Bottom 3rd bolt on the alternator is a difficult one because the A/C lines are in the way. A ratcheting wrench works well here. Don't forget to unplug the electrical connections on the alternator.
3. You can now access and see the thermostat, yes it's all one long piece including the metal pipe looking part. Remove the thermostat bolts, one on the bottom is tricky, need to get under the vehicle to see it better.
4. Let all fluid out with bucket underneath.
5. The o-ring is a PIA on the new thermostat, use some gasket maker or sealer to kind of "Glue it" in place so it's properly in the correct position.
6. Replace with new one in reverse order.
While you're in there, you can change out your coolant temperature sensor too if you want, it's about $37 and is about 2" above the thermostat. Just inplug the electrical connector and unscrew the sensor from the block...easy.

The reason I mention the coolant temp sensor is that sometimes people will put the thermostat in to find out it's still not running at 210 degrees. When finished, temp gauge should read dead on 210 which is normal operating temperature for Envoys and Trailblazers.

Hope this helps, good luck!

Remove the alternator, drain radiator, remove the drivers side tire to gain access.

Parts Approx...... $70
Labour ...............2 hour. May take up to 8 Hours. It Depends on your experience how fast you get the job done.

Location. its located somewhere behind the alternater. so you must romove alternator. (Ac Compressor removal is optional)

1. Disconnect Battery Cable,
2. Raise the car on Hoist Or Jack up
3. Drain Coolant from Radiator
4. Remove Right Front Wheel and Splash Shields
5. Remove Belt and Tensioner
6.Remove Dip Stick Bolt + remove Dip Stick tube. ( cover dip stick hole so that coolant dosunt get in to oil pan) or (Do an oil change if coolant gets in to oil pan)
7. Remove Lower Alternator Bolt
8. Remove Uper Alternator Bolts (Access from the Hood).
9. Remove All the Connections to the Alternator and Ac.
11. Remove Thermostat housing bolts
12. Remove thermostat

to installl Reverse the procedure and at the end open the bleeder valve and pour in coolant slowly in to the reservoir

Start Engine

add coolant and bleed the air out.
let the thermostat valve open and cerculate coolant untill Rad cooling fan cycles.

thats it you are done!!

Just replaced my 2005 envoy thermostat, and it might be the same. Here is what I had to do.
1. First, the thermostat was located on the driver side behind the alternator. The hose comes from the bottom of the radiator, not the top like most vehicles I have worked on.
2. You need to remove the belt by taking tension off of it. The tensioner is on the passenger side, under the steering pump. I used a socket wrench, placed in in the square hole on the tensioner, attached a pipe for leverage to the socket, and pulled until the belt was loose.
3. Next is removing the alternator. Best if you first disconnect your battery, as there is a hot line to the alternator. Disconnect the wiring harness located on the top. Remove the bracket bolts that hold the alternator. You might want to place something under the alternator to prevent it from damaging the small wheel under it as it will be heavy and free after you remove the bolts. I also removed the bracket to get full access to the hose and thermostat. There is a red wire that connects to the alternator, remove it and now the alternator can be pulled out and set to the side.
4. Once all this is out of the way, you can remove the hose to the thermostat (I assume you have drained the radiator).
5. Remove the two bolts hold the thermostat to the engine.
6 Replace the thermostat and reverse the order of the steps above.

I read that some can do this without removing the alternator or just loosening, but it is tight and makes the job a lot harder.

Hope this helps...Joe

The thermostat on this vehicle's (the Envoy & Trailblazer are essentially the same vehicle) In-line 6 cylinder engine is on the driver's side of the engine, below & behind the alternator. It is not found in the typical location (which may be for the 8 cylinder engine), in an answer above.

Most directions outline removal of the alternator to gain easy access to the hose clamps, bolts and housing of the thermostat. I have located a site that after free registration, has detailed instructions on how to do this job, and the link below that describes an easier method with photos. Check out:

http://forums.trailvoy.com/showthread.php?t=54225&highlight=replace+thermostat

Jack up vehicle and support with jack stands
Take off wheel
remove brake caliper
remove caliper bracket
remove rotor
remove axle retaining nut cotter pin
remove axle retaining nut
remove axle from output flange
pull axle out of vehicle
hub and bearing assembly held in by 4 bolts

installation is reverse of removal. You will need to find out the torque specs for the axle retaining nut.