that's a thermostatic Ranco valve - GM quit using the capillary tube on them cuz they don't last - you should be able to find a manual one, or have that one rebuilt, this guy can fix you up , might even have a new one

Bill


Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics