Hi all, the plastic heater control valve broke off and now we can't attach the heater hose (probably from another GM car).
I managed to find part 74691 as per Rockauto but the garage says it is now blowing smoke.
Can someone please explain what the little screw on the side is for as I have nothing to attach to it or can I just cap it off? Thanks and regards!

 

The threaded port on the side is for Comfortron cars and not used on Tempmatic or Custom AC cars. A sensor screws into it that tells Comfortron the coolant temp is high enough for Comfortron to start controlling temperature automatically and allowing it to start. No need to worry with it or cap it off; it's just a parts consolidation thing.

What did the shop mean "blowing smoke"? As in steam out the tailpipe, oil smoke, what? I can't think of a reason changing out a hot water valve would cause either, unless you lost enough coolant to critically overheat the engine.

 

What Glenn said. To elaborate, that port on the side is a blind hole. You don't need to do anything - it won't leak.

 

The only way i can imagine heater control valve causing the engine to burn antifreeze is if the control valve bottomed out against the head, causing the manifold to “lift” and leak coolant internally. Maybe they cracked the manifold over tightening it? Maybe coolant could seep from the coolant passage into a intake runner. I admit it’s a remote possibility.

 

Thanks for reply about the screw. I just came back from the shop, forgot to take pictures but it wasn't smoke but coolant came out of the little weep holes which you can just see from the 74691.
The shop will fix it for now to use a coupler to connect the heater hose directly on the intake. Does this mean this part is faulty, diaphragm inside the vacuum pot broken?

 

So the problem isn’t smoking from antifreeze, but a leak? It’s common for the valve to leak coolant from the vent holes. You could replace the valve with a pipe nipple, but then the heater won’t shut off. The control valve is designed to block coolant flow to the heater core while the A/C is on.

 

And to restrict the amount of coolant flowing through the heater core so it doesn't blow out. It took me several heater cores to learn this.

 

Don't. Get another vacuum valve. Like Kenneth said, you put in a pipe nipple without the proper pressure reduction orifice, you'll be replacing a heater core. Do the job right and don't let the shop talk you into doing otherwise.

 

Thanks for the info to not install directly to the heater core unless a restrictor is installed.
The heater valves may cost $10 in the US but here in the Netherlands they cost €40 so I was lucky to find a second unit after my 3 hour bogus trip for the first one.
Here are below the broken unit which looks like from the 80's maybe someone can spot from which car? And the replacement unit which the shop twisted to get out again.

Now my next issue is that when driving back home that the ventilator was not blowing fresh air to my face but only air came out in the feet area, is this another vacuum issue or blown fuse somewhere?

 

That is a classic sign that one of the vacuum lines is disconnected from the vacuum reservoir. I am not certain about the 1976 cars, but the earlier ones were mounted on the firewall. Look for a round ball about 5" diameter; it should have two vacuum hoses connected to it. It is likely that one of the hoses was pulled off during the heater valve work.

 

Thanks it is located on the left front fender, I have checked all the lines made sure all are firmly attached but unfortunately no result and still air only at feet height.
Which is weird because everything was working perfectly until we changed the heater valve but that vacuum line is also attached, what else could it be?

 

Did you check to make sure the vacuum line is connected to the intake manifold?

 

Yes I have checked all lines and also found that the car now has rough idling about 150 rpm below stationairy so must be a vacuum leak somehwere.
There is so much vacuum on these cars it's not even funny any more
I noticed a sound somehwere which could be from that ball I will start by replacing it, but could the vacuum ball also cause the idling issue?

 

Fun71 you're brillliant!
Checked for the third time and found a hole in one of the vacuum tubes (totally hidden from view) from the vacuum ball to a nipple on the intake.
Don't think tape will help here as they are tiny, does anyone have the size for these?
I had seriously NO problems before I went to this shop..

 

The parts stores here carry that flexible plastic tubing in the Dorman or Help section, does your part of the world have that product line? If not, I’m guessing you could probably come up with something that will fit. That kind of hose has been used for washer fluid, cruise control vacuum supply, etc on countless cars. Good luck!

 

I checked the whole hose again and found a complete burned piece right upto the plug that connects to the nipple on the intake.
Fow now I have fixed it with a connector piece that I could slide over it but there is not much meat on the bone.
If someone could please help me with the Dorman part numbers for hose + connector I am sure Amazon can ship these.
Thanks for all people who helped solve it!