H-valve for LS coolant/heat

Hi- so installed the H-valve on the coolant lines that run to the heater core.
Two questions:
- I'm assuming for it to function I need to attach a vacuum line to it. Best place to pull that from?
- can anyone explain why I need it, or point me towards the info? The guy that's tuning the engine thinks it's a dumb idea, or at least not needed, and despite my searches for concise intel, I haven't found anything that really explains the function/need.

Thanks-

If you want to have a heater it is needed. I forgot where I pulled vacuum from so I looked. There is a nipple by the AC ports on the firewall that I ran a vacuum line to the back of the intake manifold. Off of that line is a tee going to the vacuum H-valve. The heater will be on full if you don't have vacuum, it fails wide open.

The heater control valve shuts off the flow of coolant going to the heater core. You need it in August and when your air conditioner is on so there won’t be a flow of heat in the cab. If you live on the north pole, by-pass it. If you live in the desert, block it off. If it’s a race car, throw it and the heater away.

You could replace it with a manual shut off valve and turn the coolant flow on from under the hood if you got caught out and needed the defroster. Otherwise, look for a vacuum hose so you can operate the heater\defroster and a/c from the dash.

As Antho mentioned there is a vacuum port on the firewall just under where the original DME boot goes in that is the vacuum source for the heater control valve. This is only on the 85.5+ cars though. The early 944 is all cable and push rod actuated.

Just take note of where you mount it, especially if its a plastic one. My original one was mounted right over the headers and it practically melted it. I have relocated it further forward, closer to the water pump to keep the radiant heat from the exhaust off it.

My original plastic H valve leaked, replaced it with a metal one, better choice.

simon123 wrote:My original plastic H valve leaked, replaced it with a metal one, better choice.

I have had two that leaked. I took it apart put some gasket maker in the leak and it hasn't leaked in two years. If/when it fails I will try a metal one.

So, even after advanced searching, I'm still confused as to how to supply vacuum to the H valve switch- I have everything plumbed except for vacuum going to the vacuum supply port that is under the AC ports on the firewall.
Do I pull vacuum off the LS motor? Where/how?

Thanks-

I converted mine to cable actuation, but I haven't found a source for decent control cables. I used a choke cable for a lawn mower, but it corroded badly enough in less than a year that it doesn't work anymore. I need to figure out where to get lengths of the same type that came from the factory.

Are the stock valves on the late 944s still cable-operated or did they switch to vacuum?

Jasonscottw wrote:So, even after advanced searching,  I'm still confused as to how to supply vacuum to the H valve switch- I have everything plumbed except for vacuum going to the vacuum supply port that is under the AC ports on the firewall.
Do I pull vacuum off the LS motor? Where/how?

Thanks-

I get vacuum from the back of the intake manifold. Most of them have a port on the back for power brake vacuum. You can reduce this port down to the small vacuum line need for the heater valve.
You need to run a vacuum line from the intake to the Porsche check valve that was original on the car. From there it will go to a tee where one leg of the tee goes to the vacuum reservoir and the other leg of the tee goes to the top port on the firewall. The bottom port on the firewall goes to the heater valve.

The link below has some decent pics.
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Thanks Hotrodz- I get it now, but:
-having a hard time locating the brake booster port on the manifold. Is it separate from the PCV
system, or is it the smaller port/nipple on the same "stalk" that the PCV gets routed to at the back of manifold?

-I of course threw out that vacuum reservoir in my overly zealous stripping of the engine bay.
Do you think i specifically need a Porsche vacuum reservoir, or are they pretty universal with an in and an out? And do I need the reservoir at all?

Thanks for the help-

EDIT: I think I'm in error thinking the PCV got routed to one of the ports on the back of the manifold.
Diagrams show one port being for HVAC control, and the bigger one for brake booster.
If I use the brake booster port, do I just block off the HVAC control port?
Or can I use the smaller HVAC port, and block off the booster port.

Still same question as to vacuum reservoir though-


Thank you-

Jasonscottw wrote:EDIT: I think I'm in error thinking the PCV got routed to one of the ports on the back of the manifold.
Diagrams show one port being for HVAC control, and the bigger one for brake booster.
If I use the brake booster port, do I just block off the HVAC control port?
Or can I use the smaller HVAC port, and block off the booster port.

Still same question as to vacuum reservoir though-


Thank you-

Only the GM brake booster got attached at the back of the intake. Some intakes had a small vacuum port as well. You can use either one for your vacuum source.

As far as the reservoir goes, it just has one port on it. Anything that will collect vacuum will work.

This is typically what the back of the intake looks like. The map sensor is on the right and the vacuum port on the left.

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This one has the little port on the right as well. Either one will work.

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Thanks hotrodz-

Your pic shows exactly what I have, one small, one big under the sensor.

Now I'm on the hunt to find some brand of vacuum reservoir that is cheaper than the Porsche-
Do you think this would work?

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And can fuel line be used for vacuum lines? I've found many different opinions when researching-

Many thanks-

Once I get this resolved I get to fire it up

those vacuum cans are pretty big. if i were you id just hit up the auto wreckers and grab one from a random car. various gm cars from the 90s have a small vacuum "ball" in them.

another thing to consider is you dont actually have to have the vacuum canister for the valve to function. the vacuum line has a check valve in it, and vacuum is used to turn the heat on. 99% of the time you turn the heat on your at idle or cruising speed where the engine has vacuum, it will work normal. turn the heat on while at high load and its not going to turn on, cause the engine isnt making vacuum (who turns their heat on while rowing gears anyways?). however once the heat is on the check valve holds the vacuum on the valve (unless you have a vacuum leak in the heater system) so its not going to shut off.

Jasonscottw wrote:Thanks hotrodz-

Your pic shows exactly what I have, one small, one big under the sensor.

Now I'm on the hunt to find some brand of vacuum reservoir that is cheaper than the Porsche-
Do you think this would work?

[You must be registered and logged in to see this link.]

And can fuel line be used for vacuum lines? I've found many different opinions when researching-

Many thanks-

Once I get this resolved I get to fire it up

You can buy the Porsche reservoir from ebay cheaper than that one at Summit. Or like Sharkey said, hit up a wrecking yard for the ball type that most gm and fords have on them.
You can get plain rubber vacuum hose from any parts store. And it will be cheaper than fuel hose.