Manifold Heat Valve - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Fred n' Ethel · 05-14-2018, 09:41 PM

54' - 235
Was frozen up, free now, spring in good shape.
How it works ? ? ?
When engine is cold, the spring holds the shaft in the CW direction when looking from the front of the engine. The counterweight is on top and a little to the right.
Bring the engine up to temp., and nothing changes.
The spring is preloaded about half a turn.
I understand this is to preheat the intake, then open up (actually close) to divert the exhaust away from the intake when the bi-metal spring gets hot.
Am I on the right track ?
Thanks for any help here.

**mr48chev** · 05-14-2018, 11:35 PM

Yep, in the cold position it diverts the exhaust to heat the intake and when it heats up the valve opens up to the exhaust go down the pipe in a more direct path.

Fred n' Ethel · 05-15-2018, 05:32 AM

Thanks for the verification !

Now, the crux of the post is:
When the engine gets warmed up, nothing happens at the valve.
Maybe the spring is wound too tight ?
Is hot at idle enough to swing the valve ? or, is it only hot enough under load to swing it ?
I would think it should close and stay closed as long as the engine is at op temp. Like the spring for the choke.

Thanks again,

1project2many · 05-15-2018, 06:47 AM

As the spring heats up the metal gets longer. Combined with the weight this allows the valve to open. If the manifold is hot to the touch the valve should be open although it might not be fully opened. There wasn't a bunch of spring pressure unless the engine was pretty cold. If you tried to "wind" a loose spring you would usually damage the spring and it would never work correctly again.

Foot Stomper · 05-15-2018, 08:01 AM

Flip the spring over. Perhaps it's on backwards?

Volfandt · 05-15-2018, 09:37 AM

On my 49 216, once the engine heats up and the spring relaxes it doesn't fully close the heat valve, instead it uses the increase in rpm and exhaust gases to fully close it. I can watch it move when doing this. When the engine is cold and the spring tension is taught the flapper doesn't move no matter how high I run the rpm.

The service manual shows it pretty good. Be sure to go to the next page as it shows how the spring should be installed. http://chevy.oldcarmanualproject.com...47ctsm0604.htm

Dave

_Ogre · 05-16-2018, 12:39 PM

cut it out. they rarely work right after a few decades

05-14-2018, 09:41 PM	#1
Fred n' Ethel Registered User Join Date: Feb 2018 Location: Eastern WV Posts: 161	Manifold Heat Valve 54' - 235 Was frozen up, free now, spring in good shape. How it works ? ? ? When engine is cold, the spring holds the shaft in the CW direction when looking from the front of the engine. The counterweight is on top and a little to the right. Bring the engine up to temp., and nothing changes. The spring is preloaded about half a turn. I understand this is to preheat the intake, then open up (actually close) to divert the exhaust away from the intake when the bi-metal spring gets hot. Am I on the right track ? Thanks for any help here. Last edited by Fred n' Ethel; 05-14-2018 at 10:56 PM.

05-14-2018, 11:35 PM	#2
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 15,710	Re: Manifold Heat Valve Yep, in the cold position it diverts the exhaust to heat the intake and when it heats up the valve opens up to the exhaust go down the pipe in a more direct path. __________________ Founding member of the too many projects, too little time and money club. My ongoing truck projects: 48 Chev 3100 that will run a 292 Six. 71 GMC 2500 that is getting a Cad 500 transplant. 77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.

05-15-2018, 05:32 AM	#3
Fred n' Ethel Registered User Join Date: Feb 2018 Location: Eastern WV Posts: 161	Re: Manifold Heat Valve Thanks for the verification ! Now, the crux of the post is: When the engine gets warmed up, nothing happens at the valve. Maybe the spring is wound too tight ? Is hot at idle enough to swing the valve ? or, is it only hot enough under load to swing it ? I would think it should close and stay closed as long as the engine is at op temp. Like the spring for the choke. Thanks again,

05-15-2018, 06:47 AM	#4
1project2many Registered User Join Date: Mar 2012 Location: Lakes Region NH Posts: 3,200	Re: Manifold Heat Valve As the spring heats up the metal gets longer. Combined with the weight this allows the valve to open. If the manifold is hot to the touch the valve should be open although it might not be fully opened. There wasn't a bunch of spring pressure unless the engine was pretty cold. If you tried to "wind" a loose spring you would usually damage the spring and it would never work correctly again. Last edited by 1project2many; 05-15-2018 at 06:57 AM.

05-15-2018, 08:01 AM	#5
Foot Stomper Registered User Join Date: Oct 2015 Location: Calgary, Alberta Posts: 1,252	Re: Manifold Heat Valve Flip the spring over. Perhaps it's on backwards? __________________ So when is this "Old enough to know better" supposed to kick in? My 1959 GMC build thread http://67-72chevytrucks.com/vboard/s...d.php?t=686989

05-15-2018, 09:37 AM	#6
Volfandt Registered User Join Date: Jan 2018 Location: Knoxville TN Posts: 87	Re: Manifold Heat Valve On my 49 216, once the engine heats up and the spring relaxes it doesn't fully close the heat valve, instead it uses the increase in rpm and exhaust gases to fully close it. I can watch it move when doing this. When the engine is cold and the spring tension is taught the flapper doesn't move no matter how high I run the rpm. The service manual shows it pretty good. Be sure to go to the next page as it shows how the spring should be installed. http://chevy.oldcarmanualproject.com...47ctsm0604.htm Dave

05-16-2018, 12:39 PM	#7
_Ogre Registered User Join Date: Jan 2012 Location: Motown Posts: 7,680	Re: Manifold Heat Valve cut it out. they rarely work right after a few decades __________________ cool, an ogre smiley Ogre's 58 Truk build how to put your truck year and build thread into your signature shop air compressor timer



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