Re: What is this?
Kinda hard to tell what it is hooked up to, but based off the type of connection and tubing, that is for some sort of aftermarket pressure sensor...that or I suppose it's possible that line actually flows something through it, but to what? lol I know the heater line shutoff valve is right there. If it's tied into there somehow then I would suspect it was for a cooling system pressure gauge. Without more detailed pics it's impossible to say for sure.