Vacuum water valve

[72 K20]

Could someone tell me how many inches of vacuum are required to open the water valve ?
My truck only carries around 10 inches of vacuum at idle and I am not getting much heat in the cab .
Motor runs right around 195 - 200 degrees'.

New heater core , valve, and hoses . One hose is hot ,, the other is warm .

I hooked the valve directly to manifold vacuum and no change.

Wondering if I got a bad valve ............... tomorrow I'm going to try taking the valve out and just splicing the hose together .

Give me some things to check , tips , info !

Thank You ! Tom

[Stocker]

I've had a couple of those valves go bad recently. Replaced it the first time with a new one from NAPA. It worked for a while, then failed the day the truck went into the body shop, where it still is. I haven't checked my vacuum but everything else works OK so I think it's a bad valve again. I've heard the quality isn't too good on those. The valve might be sticking inside. Try rapping on it with a wrench or something..... maybe it will open up and you'll have heat.

[geezer#99]

Only 10 hg of vacuum!
What have you got in the motor for a cam?
Should be nearer 20 where you live.

[72 K20]

290 hp crate 350 ,,, it says right in the instructions that it wouldn't have a lot of vacuum at idle. My '46 with a 270 comp-cam 350 runs around 15 at idle. I'm pretty sure I have a bad valve .

[geezer#99]

While we could have a long discussion on the inadequacies of your crate we’ll leave that alone.
You need heat!
Have you tried applying more vacuum to the valve to see if it does work or not.
A little lung power is all you need with a longer vacuum hose.
You should be able to pull 20 hg of vacuum with your lungs.
Could save time buying a new valve.

[LockDoc]

Originally Posted by 72 K20

290 hp crate 350 ,,, it says right in the instructions that it wouldn't have a lot of vacuum at idle. My '46 with a 270 comp-cam 350 runs around 15 at idle. I'm pretty sure I have a bad valve .

I'm wondering if a small vacuum tank would help. I have a small round one from an S-10 that I saved because I was going to build a 383. I don't know if something like that would help in your situation or not.....

LockDoc

[72 K20]

Lock Doc It has the plastic one about the size of a softball . I'll bring the mity-vac in and try that ,,, otherwise it's getting a ball valve in place of it. I'll shut it off in the spring .

[davepl]

In either 1970 or 1971 they changed. They simplified the heater control from a 3-port to a 2-port and reversed the operation of the vacuum diaphragm from suction opens to suction closes, or vice versa.

So if there's any chance you've got the wrong diaphragm or heater control, or they could be mismatched, that'd be why...

[K20super]

I put one of the aluminum heater cores in mine and it did the same thing. I went and had my brass copper redone and it works great again.

[72 K20]

K20super That is a very real possibility ! It is an aluminum heater core .

[clay68c10]

The heater valves I’ve dealt with all close with vacuum applied. Pull off the vac hose and it should be full flow and full heat.
Have you checked the blend door operation?

[72 K20]

I have heat now ! Water valve is junk ! I warmed the truck up and no heat in cab ,, applied 20 inches of vacuum , still no heat . shut the truck off ,, took vacuum valve out and put a ball valve in . Started the truck and have great heat now .

Some of these valves close with vacuum applied and some open with vacuum applied.
I have experienced both types and normally a little vacuum hose re-routing will take care of it if it is opposite .

Thank you everyone ! Tom

[Andy4639]

I haven't had a vacuum valve in my truck for 20 years. The manual valve works great. It evens helps the ac in the summer.