Thats a vacuum hose, mine dissapears into the fire wall on my 78 ,the other end goes to the hot water control valve,Ron 7838866.2862268519

I been waiting on you to chime in on this, Ron!!!

Does you car have a line(plastic) going to the exhaust heat riser valve, and what color is it?Adams' Apple38866.3691319444

Under the hood....Yep...

opposite side though....Sorry...

You da man Ron.

Wull...thanky....thanky very much...

Okay, I'm a little late (alright, a LOT late) chiming in on this issue--but
now I'm finally back from my travels above and beneath the shark today.
While there I did investigate the yellow line further, and discovered that
(you were right, Joel) it is a vacuum hose.

And now Ron 78 has shed more (sun)light on this mystery.

Here's the deal: I traced three lines--two black, and the yellow--
emerging from a grommet on the firewall on the passenger side, just
behind and to the right of the distributor. One black line is routed to the
left, to a "T" junction over by the throttle cables; all the connections there
are vacuum hoses. The other black line is routed back through another
grommet in the firewall--actually, in the windshield wiper cavity--travels
all the way over to the far right passenger side, and disappears into the
passenger side cabin.

And the yellow...the yellow...the lousy lemon line.....just goes nowhere.

Sorry Ron, but I don't know where the hot water valve is on this car. I'm
certain that it's someplace on the right rear side of the engine
compartment, but I'm waiting on my various manuals to arrive, so I'm
flyin' blind here.

There IS a capped vacuum outlet on a junction on the intake manifold,
just below the air cleaner, and to the rear of the carburetor. But it appears
to be too large a nipple for this yellow hose. For the life of me, I can't see
any other nipple in the engine bay that could accomodate this very small
diameter vacuum line.

So, any directions you guys can provide as to where I plug this sucker
back in will be most welcome.

And thank you, one and all, for your knowledge and assistance...

mystery.......almost....................solved!

One black line is vaccuum source from intake(by the cables).
The other black line, that goes thru the wiper compartment, is for the air door actuator in the pass. side lower plenum. It allows air to recirculate in the max. position A/C.
If your car does has not water shut-off valve, then just plug/cap the yaller line. If you someday wish to install the valve, you'll know where the vac. comes from...that lousy lemon line!!Adams' Apple38867.2068981482

Ron & Joel, you've solved the dilemma. Now I know what this trio is
intended to do--thank you, again, for all your persistence and assistance.

And now that I know what I'm looking for--the hot water valve--I'm
going to examine everything in the vicinity all over again and see if I can
replace Old Yeller on it. It's entirely possible that a PO removed the
valve--this car is a strange mixture of virtually-untouched-since-'78
items, and a let's just-yank-this-thing-off-and-discard-it mentality (if
you can call it that to begin with!)

Cabin comfort--temperature and air flow both--is very important to me,
so the vacuum systems that enable full factory-style ventilation rank high
on my list (which is growing longer by the day, but that's something that
everybody on this forum can relate to, I'm sure!)

Hopefully I'll be able to report back later on this thread with a small
success story--and then we can call it "case closed"...