Hi guys...been checking this forum for a while now but can't seem to find an answer to my problem from the existing posts. This is a bit of a long story here...bear with me while I give some history of what's going on.

2002 Grand Prix SE, 83K miles. Have had this car for 4 years and it's ran like a top. No major problems that I couldn't fix. Regular full-service oil changes at an oil change place (no free plugs here).

A couple of months ago (summertime in TX, hotter than hell), I started seeing the temp gauge creep up quickly (upwards of the red at the top of the gauge) when stopped or crawling in traffic. Once I got going the temp came back down quickly. After a couple of days of this I checked the coolant and had to add about a half gallon. Quite a lot, I think. I checked around where I park at home and work and there are no coolant puddles. After I added the coolant, the temp gauge never gets higher than 1/2 way up the gauge (which is good) but I started smelling hot coolant after the car has been running for a while. I checked to make sure the radiator cap is on tightly, the top of the reservoir tank is on, upper and lower radiator hoses and water pump bypass hose. The oil is clean and there is no smoke coming from the exhaust.

I can't see where any coolant is leaking but It's really bothering me because I have never smelled hot coolant unless the radiator cap is off. I'm even more concerned because this thing is still running like a top.

I've also felt around the bottom of the water pump for the weep hole but can't seem to find it. Where might that weep hole be?

BTW, in the pic below where does this plug go? I found it when I fixed a burned-through vacuum hose (battery leaked). I don't remember unplugging it and I can't see an obvious place where it goes. It might be related to the problem at hand.

Anybody got any ideas on what the plug is and what the deal with the coolant smell is? Has anyone had a similar problem? Maybe a t-stat issue?

Thanks in advance,
Lerxst

(Sorry for the monster-sized image...lower-quality pics just didn't show the plug that well.)