Is the heat riser open with vacumn or without vacumn??? This is a St louis build. Thanks

 

All of the ones I have dealt with were OPEN normally. IF you are wanting to get it to operate CORRECTLY, It requires one of the ported vacuum swtiches that are located in your thermostat housing or top of the intake area by the thermostat housing to ALOW vacuum to pass through the switch and go to the heat riser. When the water temps gets high enough, it will CLOSE the ported vacuum switch and supply vacuum to your heat riser WILL stop and allow it to open....due to it being spring loaded.

"DUB"
"DUB"

 

Mine stays down all the time....BUT, if I disconnect the vacuum hose it pops up immediately.....Does that mean that I need a new Thermal Vacuum Switch ? I am beginning to think so.

 

Your heat riser is differeent than that of a 1981. It is connected to a rod that connects to the vacuum diaphram. I for the life of me can not remember which way yours is positioned when the engine is off and no vacuum is applied to the vacuum diaphram.

Unless someone else knows and responds, you will have to CAREFULLY remove the three nuts that hold your exhaust pipe to the exhaust manifold and see what position the heat riser is in when the pipe is out of the way and the heat riser is pushed back up into correct position. SORRY about the "brain fart". You may also need to loosen the other side exhaust nuts at the manifold also so you "y" pipe will drop down far enough.

OR...call a supplier that has one in stock and have them describe to you the position of the ball end that the rod attaches to so you can identify which position your is in WITHOUT the engine running.

When you can identify which way your is when the engine is not running, respond and I can help you further. GM will switch things around from time to time and it is hard for me to remember EVERYTHING. GM can never leave something the same...and it is hard to remember when they changed designs. I know your heat riser is from 1975-1979 ... but other than that I would have to see one installed to tell you how the ported vacuum switch controls it...sorry about that.
"DUB"

 

i have mine disconnected and it stays open. BTW i have an 81

 

Mine now works....It must have been sticking after 15 years of being garaged and really not driven.
When the car is warmed up , the rod rises , opening the butterfly.When the car is cold it is in the down position, which means the butterfly is closed.
I basically worked the connecting rod up and down until it moved freely.Then I ran the car through a few hot cold cycles and watched it very closely.....It now works......Yayyy !!

 

Without vacuum....Pull the hose off and it pops up. When my car is fully warmed up, the rod is in the up position, the heat from the TVS closes the switch, stopping the vacuum.

 

Thanks