Alright, I spent most of today replacing my heater core (took a long time ~7hrs, but went OK), only to find that my old one was fine (but full of nasty old coolant). I checked the vacuum line to the Heater Control Valve (wtf- is this valve thing really necessary?) - zippo vacuum when heat is turned on inside car!

So, I pulled another vacuum line(it was going into intake, I think) over to test the valve, and hooked it to the HCV - I could feel the HCV "click" a little, but the hose to the heater core still didn't really get hot (I guess it's not opening enough?).

Sooo, no vacuum to the valve and a bad Valve... (does this sound right? I'm an amateur.) -- how do I fix the no vacuum problem? Or should I give up and buy a T-connector to permanently link up the 3 hoses that go to the HCV?

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Joel Geerling
Red 91 Formula, 305 TBI
Edelbrock 14x3 open element
TBI 1/2" spacer
Hypertech street chip
Crank/Alt Pulleys Exhaust is next!

 

Joel,

There is a ported vacuum switch incorporated into the heater/AC controls. The slide selector you move to operate the various functions of the system is actually operating this ported switch.

There is a small vacuum line connected to this switch, and several small vacuum lines from this switch connected to several servos (insluding the hot water valve) that receive vacuum at the appropriate position of the selector.

There are a couple of possibilities. Either there is no vacuum supply to the switch, or the vacuum line for the hot water valve is damaged, or the ported switch is failing.

If there is no vacuum supply, all air from the system will exit the defroster vents (the default position for the system). If moving the controls from HEAT to VENT to DEFROST alters the flow of air to the correct outlets, at least you have vacuum to the switch and most functions are working. If moving the selector does nothing, you need to track down the vacuum line through the firewall and under the dash.

If everything else is working, you need to remove the dash control and find the vacuum line for the hot water valve. Trace the line back through the firewall and look for damage.

You should also test the vacuum at the ported switch to assure that the switch is functioning. If there is little or no vacuum at the ported switch fitting, you may need to disassemble and clean/regrease the switch body.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

OK, I'm stupid (gotta get that out there 1st). Thanks for the reply Vader... the vacuum works fine -- there's vacuum when the heater is OFF, then no vacuum when on (I had thought it was the other way around, but I messed with the valve and that's how it works).

So, here's the funny (for the reader, not me) story about what the problem was. I got so pissed at this problem, that I took the HCV out and tested it with water and sucking on it - it worked fine. So, I start testing flow through the heater core and hoses with water from my garden hose and I check all the hoses/clamps... all checks out, but still no heat - no flow through the heater core!! Finally, I take off the radiator cap and I notice that (with the heater on, HCV open, engine hot) no fluid is coming in from where the hose back from the heater core clamps on...I pull the hose there and what do you know? The F***in little screw-in plug (I think radiators ship like this, with the plugs, so you can pressure test them 1st?). So I take the plug out and everything is fine, plenty of flow, heat in the car.

I guess the previous owner had the radiator replaced and never found out about the plug...

I basically replaced a perfectly good heater core and wasted a day and a half for something that took about a minute to fix... grrrrr. Oh well, at least I'll never have to worry about replacing the heater core. And it's fun to get in touch with the ***** of your dashboard every once in a while.

 

Having just replaced my own heater core, I feel your pain.