Heat riser valve operation

[Scooter76]

I did a search & could'nt find an answer. On my 76 the exhaust heat riser valve is vacum operated. My ? is when vacum is applied does this open or close the valve? I think the temp switch on the water neck is defective because it is allowing vacum to the valve at all times whether the motor is cold or hot. Thanks in advance.

 

[AKRAY4PLAY]

if i remember right, vacuum opens the flapper after the temp is up. Hopefully some will have a deffinate answer for you. Brian

 

[Enigma]

It's the other way around.

When the TVS (Thermal Vaccuum Switch) is cold it allows vaccuum to close the heat valve. The actuator is spring loaded.

As the Engine warms up, the TVS slowly closes and cuts off the vaccuum and opens the valve.

TVS is usually mounted on the thermostat housing.

Enigma

 

[Scooter76]

So when there is no vaccum to it , the valve is open thus allowing exhaust to flow through the exhaust pipe as normal. This is what I wanted to hear. I really do not want to drop the pipe just to see the position of the flapper valve. I may sweet talk a guy at work to get the infrared camera on it to see if it is open or closed by the heat signature in the exhaust pipe. Thanks for the input.

 

[Pancho]

I hope this isn't too much off the original subject, but....

Does anyone know what determined whether the heat riser was vacuum vs. direct thermostatic?

My 76 (federal L48) has a non-vacuum heat riser; a coiled thermostatic spring relaxes as it gets hot and a counterweight opens it.....

Our 79 (also federal L48) has the TVS-operated vacuum heat riser.

I noticed on Scooter76's L48 you have the vacuum operated type..... is your's CA-emissions or federal ?

 

[Enigma]

Scooter.

In the morning raise the hood and watch the actuator when someone starts the car.

With the car running the valve will be closed. As the engine heats up you will see the actuator move to the open position over about 3-5 minutes.

Enigma

 

[Scooter76]

Mine is a California car. I guess that is the diffrence. Pancho, does your car have an air pump? Mine does/did. If the federal 76's did'nt have one, being in texas I'm hopeing that no one will notice it missing. As far as the hear riser, I started it cold yesterday & it was getting vaccum & I asume the valve closed, when the motor got up to operating temp it continued to recieve vaccum, so I suppose it has been staying in the closed position all the time. I guess the vaccum switch is defective. I plan on just leaving tha vaccum line unhooked & plugged for now. I need to visit my next door neighbor who is a 30yr GM mechanic. Thanks for all the help.

 

[clar2001]

Mine is a Ca. car to 75 won't half to smoge it agin becouse it will be thirty years old. Question is do I half to leave the heat riser air pump cat. so on on the car...................

 

[Pancho]

Scooter76:
Mine never had an AIR pump, and it's not listed on the underhood emissions sticker -- see if it's listed on your sticker above the brake booster (mine was falling apart and I got a new one at ZIP)

clar2001:
My understanding is that even when they get old enough to bypass the sniffer test, they still do a visual inspection for tampering. At least that's the way it is in NC -- all original equipment has to be on, they just don't check that it's working...hmmmm.

From what I remember CA uses a book that lists what equipment was originally installed..... and at one time a special book for Corvettes.