What is best way to install thermostat on 2002 trail blazer 4.2 liter. should i remove the alternator, or should i go though the wheel well?

4 bolts hold the shroud on. 2 on bottom,2 on top. Leave on in on the top until you remove the clutch fan. Clutch fans are a pain to get off with out the right tools. Once off, remove the bolt from the shroud and pull them out together. Remove the serpentine belt. Remove the pump.

It's usually on if the alternator isn't charging. You'll have to connect a voltmeter to the battery terminals. If it reads 14 - 14.5 volts you'll have to look elsewhere. If it reads below 13 volts the charging system is not working. The alternator is quite often the problem.

the termostat on your 2004 trail blazer with the 4.2l in line six is located under neath the alternator you have to remove the alternator to remove the termostat.

Using a ratcheting box end wrench works for me and many other TB and Envoy owners. Difficult little bugger with those lines in the way. Use the same method for the bottom bolt of the thermostat, same problem, IF that's what you're trying to get to.

Remove the alternator and the bracket. Easier to take off if the drivers front tire is off. Have fun with the 3rd bottom bolt on the alternator, it's a blast trying to get it off.

While you're removing the thermostat, you might as well replace the coolant temperature sensor like 2" above the T-stat. It's around $35 and very easy to do. The sensor screws into the block and has a wire coming off of it that will need to be unpluged to remove.

I removed my tire and alternator to have easy access to both bolts.

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

approximately 5 litres

Usually found either on thermostat housing or close by on the lintake manifold. Usually can be removed with a regular wrench.

you may have fried the alternator...try letting it charge for awhile about 20 minutes