why cant i figure out what this is?

the previous owner of my car updated the cooling system so well, i haven't had to think much about it in my 2.5 years of ownership.

however, now i appear to have developed a coolant leak.
its coming from what appears to be some sort of filter or valve (?) on the coolant hose that goes across the firewall.
its the black circular plastic piece in the top left of one photo and center right-ish of the closer-up photo.

i can't seem to identify it in any of the parts catalogs. but thats partially because i dont know exactly what i'm looking for.

any help?
thanks.

 

That looks like an aftermarket heater valve. Its the valve that turns off the hot water to the heater core when AC was turned to full cool.

The original part was metal and mounted to the firewall. It was part number C41051.

Mark

 

yup, that was exactly it. plastic replacement piece from the local pepboys.

thanks!