I had a similar "is it the HG" scare on my 80 recently. I Smelled coolant a few times and noticed that the 80 was slowly loosing level in the coolant overflow bottle. Drug my feet on looking as I didn't want bad news but was glad to find that it's just the aftermarket heater control valve leaking
I think it is a "four seasons" heater control valve purchased about 5 years and 60k miles ago. OE will go in there and should last much longer.
