[kdrolt]

It's both a T-fitting an a coolant flow restrictor. The resistriction is to prevent high volume (or pressure) water flow into the heater core at high rpms. For this to work properly, you need (a) an orifice restriction going to the heater core and (b) an alternate flow path upstream of the orifice that can reroute excess coolant (that's why there is a "T").

The one shown in the pix is for a 94-96 9C1, as evidenced by the 1T1 silicone rubber (green colored) hoses.

The plastic T/restrictor often leaks as it starts to crack, so a brass T replacement is a cheaper solution than buying a new one. IIRC there is also a pn for the plastic T-fitting/restrictor so you don't have to buy the T/restrictor plus crimped fitting and short green hose ($70+ if I recall right). You'll have to search on the ImpalaSSforum for it. HTH.