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Discussion Starter · #1 ·
The 84 318 has a vacuum operated heat riser valve on the passenger exhaust manifold.

Is it opened (straight through) or closed (bipassed through the intake manifold) when vacuum is applied?

Thanks
 

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Are you sure it is vacuum operated?  I have only seen mechanical operations on those.
 

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there normally closed and open at a certain temp by vacuum
 

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Interesting.  Would you have an image of one?
 

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I've played with this and got mixed answers in the past also, before I got a Factory Service Manual.

from my 85's FSM,
pay attention to their use of right and left, apparently as viewed from the rear of the engine

   "The power heat valve, located between the right exhaust manifold and exhaust pipe, is a vacuum operated device which directs the majority of exhaust gas flow through the left-hand exhaust manifold.
   An engine mounted coolant switch (CVSCO) controls the manifold vacuum signal necessary to activate the heat valve.  At coolant temperatures below a predetermined level, manifold vacuum is applied to the heat valve and all exhaust gas flow is directed to the left-hand exhaust manifold.  Above this temperature no vacuum signal is applied to the heat valve and exhaust gas flow proceeds through both the right-hand and left-hand manifolds."

"CVSCO" is a Coolant Vacuum Switch Cold Open

so the power heat valve closes when vacuum is applied
 

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l was thinking like the old style bymetal spring ones that are closed when cold and as the bymetal spring heats it opens the valve
 

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I've just removed the exhaust manifold from my '84 and I was puzzled by this set-up as well.  I think my words were "that's one of the dumbest things I've ever seen".

Between the bottom of the passenger-side exhaust manifold and the tail pipe is a vacuum-operated butterfly valve capable of completely blocking the flow of exhaust to the tail pipe.  Its a 3/4" thick hunk of cast iron with a vacuum motor sticking out of one side.  When it is closed, the only egress for exhaust gas is through the 1/2" diameter pipe that leads off the manifold just above the butterfly valve and into the AIR pump (and ultimately into the driver's side manifold if the pressure is right).

Seriously?  A valve that closes off the exhaust and diverts it through a 1/2" pipe?  No way in hell that's going to survive the rebuild on my truck.

How do earlier models connect the manifold to the tail pipe?  Looks like I need to either find an adapter or take that valve body into my garage and find a way to remove the butterfly without creating a new exhaust leak.
 

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Discussion Starter · #8 ·
Donk said:
" ... Above this temperature no vacuum signal is applied to the heat valve and exhaust gas flow proceeds through both the right-hand and left-hand manifolds."
That was my interpretation as well; I was hoping I was wrong.

I'm trying to clean up the engine comparment. I had hope of hooking it up to the coolant temperature activated vacuum switches (but it's cold closed). Thinking when the engine warmed up it would open and pass through. When I saw the solenoid, I figured it was probably the reverse. It must be connected to the same coolant sensor as the gauge. Looks like I get to keep it.

What about plugging the line at the manifold and leaving it perpetually open? What would the real result be in performance? It's already slow to warm up. I'd say I have more than a reasonable probability the intake crossover is plugged with carbon. I've been thinking of switching to a manual choke, so I have a more direct hand in the warming process.

Thanks
 

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The valve is just a restrictor, it cannot completely close off the flow of exhaust.  It simply restricts the flow in the passenger side manifold so that some exhaust is forced into the aspirator tube.
 

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Copper Top said:
What about plugging the line at the manifold and leaving it perpetually open? What would the real result be in performance? It's already slow to warm up. I'd say I have more than a reasonable probability the intake crossover is plugged with carbon. I've been thinking of switching to a manual choke, so I have a more direct hand in the warming process.

Thanks
That's what I did, but I had a number of little issues going on at the same time I was doing this cleaning, I can't say what kind of effect it had if any. At one time I did run a vacuum line straight to it and there was an increase in sluggishness I thought.

Mine has the Holley 2280 carb. Once I got the electric choke functioning properly, including the vacuum pulloff and electrical parts of the choke system, and blocked off about half the radiator in cold weather, now the warm up time is acceptable. So I didn't think about the heat riser anymore.
 

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thru the 80s I ahve seen both bi metal and vacuum operated versions. my 87 B-van had the vacuum.
on the bi metal type I have removed them completely and tapped the manifold (cant remember which) either 3/8-16 or 7/16-14 and run short bolts in the remaining holes where the shaft goes thru.
Right now I run headers which makes a heat riser, moot.
 

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Discussion Starter · #12 ·
I just removed the vacuum line.  Unless I can find an open-cold / closed-warm coolant vacuum switch, I don't see another reasonable option.

It doesn't seem to effect the way it runs, but then it's 98* outside.
 
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