Link to Three Wire o2 conversion.

http://forums.s-series.org/viewtopic.php?t=12814&highlight=heated

The Thermac valve is the flaper inside the air cleaner snorkal. A round vaccume motor sits on top of it and opens and closed when intake air is cold sending hot air to engine. linkage can wear out, diapham can leak.

try to get factory KT shop manual and 87 GM Truck manual on EBAY.

3 90 Trooper V-6's, 5 speed, 4.3L TBI, 31" BFG AT's, Fender Flairs, Bored intake, 1.6 roller rockers, MSD Coil, Jacobs Omni ignition. Heavy duty torsion bars, Rancho 9000 Shocks CAN'T SPELL, TO OLD TO LEARN, GOD BLESS YOU IF YOU CAN.