Good move - great advice guys.

FYI - the typical problem people find when a thermostat goes bad is it jams OPEN of all things. This is caused by the rubber gasket that goes around the internal valve in the thermostat - it breaks down over time and can jam it open. The result being that the engine takes too long to warm up, and the computer detects that, and as a result it blanks out the digital temp display with -- --, zeroes the analog temp gauge, and disables the A/C. Eventually the check Engine Light code will be a p0128 - indicating too slow to warm up. The confusing part is, if you shut it off after a nice drive, and restart it, as it is up to temperature already, all the issues clear right up. So it often can be seen as an intermittant problem which really confuses the issue.

 

Why are you worrying about the lower hose? No need to remove it to do the t-stat or the gaskets.

The t-stat housing has an O-ring for the transfer pipe and a gasket to the block. The internal seal is part of the new t-stat. The other end of the pipe has an O-ring into the water pump.

A typical leak from the t-stat area is the plug/cap has a tiny crack in it. A new one from GM is cheap.