70 exhaust manifold heat riser/valve???

[Mike F.]

Trying to figure out if my 70 350/350 exhaust manifold heat riser valve is functioning correctly.

When the engine is cold and outside air temp is 80F, the valve weight is up (or pointed toward the passenger side of the car).
The valve moves freely with a slight push of the index finger. It can be pushed down (toward the floor) and the spring returns it back to the up position.

When the engine is hot 180F, the valve is still up and can be moved freely with a slight push of the index finger. Sometimes the spring returns the valve weight back to the up position, other times it stays down (catches a tight spot due to heat expansion) but a slight touch will free it to move back up.

With the valve weight in the up position is the valve open or closed?

What position should the valve be in when the engine is cold, and hot?

Should the valve move (open or close) as exhaust pressure builds? (Idle close, WOT open)?

With the engine hot should the valve be in just one position and not move?

I haven't had it apart yet, could the valve be gutted???

TIA,
Mike

[Rich W.]

Re: 70 exhaust manifold heat riser/valve???

I just took a long idle in a parade..mine does the same thing as yours does?? I would think it would fall once it warms up all the way..kinda like the choke box..I will ask a friend of mine today to give me his 2 cents!

[Gene M.]

Re: 70 exhaust manifold heat riser/valve???

Up when cold and down when hot.

[Alan S.]

Re: 70 exhaust manifold heat riser/valve???

Hi,
Here are a couple of pictures of the heat riser valve (cracked) from my 71 350.
First engine cold, closed (arm up), then engine at operating temperature, open (arm down).
The coil is easy to see.
What's the purpose of the spring? I can't see that it does anything?
Regards,
Alan

[Terry M.]

Re: 70 exhaust manifold heat riser/valve???

You need to rig up a closed circuit camera to look at the weight wile you are under power. It takes both the flow of exhaust gasses, as well as exhaust temperature, to open the valve.

[Robert S.]

Re: 70 exhaust manifold heat riser/valve???

Notice that the valve axis of rotation if off-set. Gas pressure on the larger plate of the valve provides a moment to open, as Terry stated.

[Joe L.]

Re: 70 exhaust manifold heat riser/valve???

Hi,
Here are a couple of pictures of the heat riser valve (cracked) from my 71 350.
First engine cold, closed (arm up), then engine at operating temperature, open (arm down).
The coil is easy to see.
What's the purpose of the spring? I can't see that it does anything?
Regards,
Alan

Alan------


The spring has an "anti-rattle" function. They weren't really that effective, though, as most of these valves rattle some from the time they are new.

[Alan S.]

Re: 70 exhaust manifold heat riser/valve???

Hi Joe,
Ah Yes!!!
As soon as I read your reply, my brain recalled that sound from many years ago!!!
Thanks!
Regards,
Alan