bypassing the heater core?

[corvdesign]

I have a "77 - There is an item that looks like some kind of vaccum module in one of the heater hoses. Can that be bypassed or do i bypass the heater core between there and the core?
thanks,
Corv

 

[George Black]

Coolant flows FROM the intake manifold fitting through the 5/8" inch hose to the lower heater core nipple; and then BACK to the water pump throght the 3/4 " hose from the upper heater core nipple. A valve in the 5/8 inch hose will not depressurize the heater core. It WILL stop the coolant flow.

The "item" you refer to is a vacuum operated shut off valve. Normally OEM installed on 1977 air cond. cars. The vacuum valve should not affect placement of any cutoff valve that you install. (The factory vac. valve is used to cut off coolant flow to heater core when AC is on.)

Check out Bob Chadwicks 1977 corvette home page for a wealth of information on this subject--and others! Bob's www. page has photos of his installed cut off valve and the OEM factory vac. valve.

 

[corvdesign]

Thanks George,
That's a big help. I have a Corvette manual, but it didn't show any information about that. I appreciate the information.
Corv

 

[BrianB]

What's that URL?

 

[Bob Chadwick]

http://home.comcast.net/~chadwick.robert/ventilation.htm

 

[pgtr]

Coolant flows FROM the intake manifold fitting through the 5/8" inch hose to the lower heater core nipple; and then BACK to the water pump throght the 3/4 " hose from the upper heater core nipple. A valve in the 5/8 inch hose will not depressurize the heater core. It WILL stop the coolant flow.

The "item" you refer to is a vacuum operated shut off valve. Normally OEM installed on 1977 air cond. cars. The vacuum valve should not affect placement of any cutoff valve that you install. (The factory vac. valve is used to cut off coolant flow to heater core when AC is on.)

Check out Bob Chadwicks 1977 corvette home page for a wealth of information on this subject--and others! Bob's www. page has photos of his installed cut off valve and the OEM factory vac. valve.

The vacuum actuated heater valve is ONLY actuated when the AC is on MAX re-circ or OFF positions on those years that have this valve. Also, as soon as you shut off the car or move the selector out of the MAX or OFF positions, the valve defaults to it's open position.

You can install a manual summer/winter shutoff valve in the vicinity of this vacuum valve or in lieu of it.

IMHO GM did a particularly poor job of engineering the MVAC system on factory AC equipped C3s. No easy or simple fixes but the summer/winter manual valve on the supply side is one of the simpler band-aides worth considering.

Good luck,

 

[lucky76]

I have a "77 - There is an item that looks like some kind of vaccum module in one of the heater hoses. Can that be bypassed or do i bypass the heater core between there and the core?
thanks,
Corv

I finally installed a cutoff in each water line to the heater on my 76 this summer. When you drive it a while or drive it and park it the heat slowly travels to the heater core without a cutoff in each line. They are about $10 at Autozone. Be sure to get both a 5/8in. and a 3/4in. manual cutoff and 4 hose clamps.

 

[pgtr]

Most MVAC engineers (for Japanese cars for example) install only a single valve on the supply side. (GM essentially skips it altogether)

Why did you install one on both the supply and return sides? Did you try it with just one side first?

 

[lucky76]

Most MVAC engineers (for Japanese cars for example) install only a single valve on the supply side. (GM essentially skips it altogether)

Why did you install one on both the supply and return sides? Did you try it with just one side first?

I thought the original vaccum cutoff was not working so I installed a manual cutoff in that line first. But I found the heat traveled down the other line to the heater core after I drove a while or parked after driving. So I bought another cutoff for the other line. There is no way hot water can get to the heater core now regardless of the temperature setting or condition of the vaccum cutoff. Also the heater core can't leak if it fails. Well it could leak but only the water already in the line.

 

[pgtr]

Thanks for the follow-up. Well many makes do not install such a double valve - just a single mechanical valve on the supply side and they dont' have a probelm w/ the heater core heat transfer w/ 1 valve shut off. I wonder what the engineers did different to avoid the scenario you just described...? Hmmmm... that's a puzzler.