Replace my thermostat

SOURCE: 2004 GMC Envoy has trouble idling

Try the fuel filter. Sounds like the engine might be running rough due to weak fuel pressure. Its cheaper to replace, especially if it's still original from 04. Try running some injector cleaner through a couple of tanks of high octane gas.

SOURCE: Replace the thermostat on 2002 GMC Envoy

4.2L Engine the thermostat and the neck where the hose goes on is one unit $25 at autozone

1. Remove the necessary coolant from the radiator.
2. Remove the alternator, Loosen the outlet hose clamp at the thermostat housing. Remove the outlet hose from the thermostat housing.
3. Remove the thermostat housing bolts.
4. Remove the thermostat housing from the engine block.
5. Clean all of the surfaces of the thermostat housing.
6. Clean the sealing surface of the engine block.

SOURCE: battery removal on a 2004 GMC Envoy

The battery is on the right side behind the fuse box. You need a metric set to undo the 3 screws. Theres one that attaches to the body, one to the bar that goes along the top, and theres one on the left side below the vacuum hose and air induction. After you've taken out those three, you should lift the whole thing out(battery and shroud) together. It's hard to take the shroud off before you lift the battery out. After that, you have to pry the shoud off the battery. The two terminals in the back make it hard. Then just go to any auto parts store and they'll give you a new one. 

SOURCE: 2003 gmc 4.2l envoy coolant temp below thermosat rating..

maybe the same problem, if its not getting to 195f its not enough.

SOURCE: replace thermostat on 2004 gmc envoy

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