292 Exhaust manifold "flapper valve"? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69_hafton · 12-21-2003, 12:42 PM

Do y'alls' with 292's have this spring loaded "flapper valve" in your
exhaust manifold? I believe my engine is a '73, so it may not have been
there on the original engine.

Mine makes quite abit of noise, especially when the idle is under 1000 rpm.
Anyone know what it does, or if I need it? The noise it makes makes it
sound like something in the engine is wrong.

**Stocker** · 12-21-2003, 01:12 PM

It's a heat riser valve. Not sure on a 292, but on V8s, it closes off the exhaust on the pass. side when cold, thereby forcing the exhaust up through the intake manifold, sort of pre-heating the intake, to help the engine run better on cold start-up. As the engine warms up, it opens and the exhaust flows out normally. I guess it must perform a similar function on a 292.

Ther're a bit expensive; some people just wire them open or remove them.

da-burb · 12-21-2003, 01:16 PM

That is a heat riser valve. First off, does yours still rotate? They are known to rust in place. You can wire it in the open position if it is stuck closed. What it does is force hot exhaust gasses through the intake system to keep it from freezing up in cold weather. As your engine warms up the thermostatic spring is supposed to allow the gate to open. It will also open under higher rpm's as you drive. They really don't make any noise. I am guessing that one of 2 things has happened. 1) it is stuck shut and your exhaust is restricted to the point of choking the engine to death, or 2) the gasket is out between the intake and exhaust manifolds and you are hearing the exhaust leak. My disclaimer....I haven't ever worked on a 6 cylinder and I am thinking that I see a gasket between the 2 manifolds in your picture above.

69_hafton · 12-21-2003, 01:20 PM

Thanks Stocker, it's good to know what it is and what it's supposed to do. I'm
guessing from what you've told me that it's not supposed to flap
open & closed at idle? The spring should slowly open it then as
the engine warms up?

I guess I'll check into prices, if they're even still availble...

69_hafton · 12-21-2003, 01:29 PM

Da burb, it's definely not stuck shut, that spring may actually be broken
or just weak, cause it moves pretty freely for a heat activated spring.
The gasket is fine also. It make a kind of rattling sound.

Cat Fuzz · 12-21-2003, 02:23 PM

I had one of those on my small block. Took it out when it broke and extended the pipe to compensate. More of a pain than anything, those are. But your is integrated into the exaust, right? I'd just wire it open to stop the rattling.

oldsub86 · 12-21-2003, 02:37 PM

It looks like the bolt that holds the two manifolds together is missing. Look at the right side of the photo. There are 2 studs on the exhaust manifold at the back (side next to the engine) that go up through the intake manifold and have nuts on them from the top. There is also a bolt that goes up from the bottom on the front through the exhaust manifold and into the intake manifold. I went through this last summer. The truck started being noisy - exhaust leak. Discovered this bolt was missing and took the manifolds off and separated them. The gasket between was about disintigrated. You don't want to leave it like that as the exhaust will ultimately damage the machined surface between the two manifolds and then you may need a new one. Machining will change the way everything lines up if they have to take much off. Cleaned it up and put it back together with a new bolt and studs. You leave it a bit loose until you have it torqued to the head so that the lineup will be correct. Bolt wouldn't tighten. Had to take it off the truck again and drill out and rethread to a larger bolt size. Touchy as there is just room for the next bigger bolt size. Seems to be ok now. Be careful with the "ears" where the manifolds attach to the head. I broke the front one trying to get it tightened back up. The books advise a torque wrench but about half of the spots won't permit a socket so you have to use a boxend or openend wrench.
As to heat riser itself - I wouldn't tie it open or take it out. The spring or the shaft may be worn. If you take the manifolds apart you can check it out. It serves a purpose and wouldn't be there if it was not required.

69_hafton · 12-21-2003, 03:05 PM

I don't think I want to wire it open. I'd like the driveability to be there
in cold weather. I think the valve leaks around the shaft, so this may
be a problem that I can't fix.

Oldsub86, I found the place for the 3rd bolt that you said yours was missing. Mine is
too. I tighted up the two bolts on the engine side a few weeks
ago and thought it was stange that there'd be only two and not at least one
on the opposite side of the manifold. Hopefully it just loosened up and
fell out. I'll try to figure out what size and length bolt I need - unless
you happen to remember.

Cast iron gets to be pretty brittle after 30 years,
so I'll have to be careful.

**Stocker** · 12-21-2003, 03:24 PM

Quote:

Originally posted by 69_hafton
I don't think I want to wire it open. I'd like the driveability to be there in cold weather. I think the valve leaks around the shaft, so this may be a problem that I can't fix.

You're right, it shouldn't rattle at idle, and as you say, the spring may be bad, or the shaft may be shot - it has spent a lot of time in a hostile environment!

As to cold drivability, I removed mine many years ago and honestly can't tell any difference. I would suggest wiring it open just to see what happens - if it's OK, leave it open; if it runs poorly when cold, remove the wire.

oldsub86 · 12-21-2003, 08:02 PM

I "think" the bolt was 3/8" and I had to go to 7/16". Not sure about length but you should be able to get a good guesstimate by pushing a wire or smaller bolt into the hole and measuring how long it is to the outer edge. ! 1/2" to 2" I "think".

69_hafton · 12-22-2003, 12:37 AM

Well, I got a bolt - 1 1/2" long and the threads in the manifold
are stripping. May have already been stripped.
I think I'm going to be better off just removing
the manifold and tapping the threads and move the next bigger bolt size
for the stripped one. It'll have to wait for another sunny day at this
point. I got a new gasket to install also.

(yawn)

JJR · 12-22-2003, 11:53 AM

I had the same bolt problem, it is fairly common. I would develope an exhaust leak from the gasket between the intake and exhaust manifolds. The threads for the small bolt were stripped. You could tighten it a little bit but could not tighten it enough. I ended up installing a heli-coil so I could use the current size bolt. As for the flapper valve, mine is siezed in the closed position. I have an extra exhaust manifold with a good flapper valve but the surface where it meets the intake manifold is warped due to rusting.

69_hafton · 12-27-2003, 10:57 PM

Just an update - I ended up removing the heat riser valve completely.
It was leaking from the shaft and I couldn't figure anyway to fix this.
I was able to remove the pressed in shaft bushing and tapped the hole
with a 7/16 x 14 tap. I'll do the other side if it starts leaking, but
I suspect it'll be all right.

The 3rd bolt on the exhaust manifold that threads into the intake -
the one that seems to strip out one everyone - would not respond to
any tapping in a possitive manner. After thinking (and praying) about
it, I went and drilled straight thru the intake, and stopped as I reached
the threads of the stud for the carb. I got a bolt from the hardware
store that would work when cut in half (all threads).

Looks pretty good at the moment, but I'll know if I forgot anything
maybe tomorrow when I put it together. Hope the exhaust flows good
without that flap in place.

All these small details take way too much time...

69_hafton · 12-27-2003, 11:07 PM

Here's another - of course I had to paint everything when it was
apart.

Hopefully this info will be of use to others. My intake was
very brittle around the threads of that third bolt.

roadking · 12-28-2003, 12:39 AM

took mine out this fall. 71 250 same manifolds, gearhead friend of mine says may cause vapor lock in the summer time due to carb over heating ( no heat deflection) if this happens he suggest putting a plate with holes drilled in between manifold to deflect some of the heat. i also bolted the manifolds togeather and had ports milled down at the machine shop, i got a great seal after that with no leaks( very quiet) just make sure you dont crack your exhaust manifold, those babys are very rare. let me know how it turns out.

12-21-2003, 12:42 PM	#1
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	292 Exhaust manifold "flapper valve"? Do y'alls' with 292's have this spring loaded "flapper valve" in your exhaust manifold? I believe my engine is a '73, so it may not have been there on the original engine. Mine makes quite abit of noise, especially when the idle is under 1000 rpm. Anyone know what it does, or if I need it? The noise it makes makes it sound like something in the engine is wrong. Attached Images

12-21-2003, 01:12 PM	#2
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,749	It's a heat riser valve. Not sure on a 292, but on V8s, it closes off the exhaust on the pass. side when cold, thereby forcing the exhaust up through the intake manifold, sort of pre-heating the intake, to help the engine run better on cold start-up. As the engine warms up, it opens and the exhaust flows out normally. I guess it must perform a similar function on a 292. Ther're a bit expensive; some people just wire them open or remove them. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

12-21-2003, 01:16 PM	#3
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	That is a heat riser valve. First off, does yours still rotate? They are known to rust in place. You can wire it in the open position if it is stuck closed. What it does is force hot exhaust gasses through the intake system to keep it from freezing up in cold weather. As your engine warms up the thermostatic spring is supposed to allow the gate to open. It will also open under higher rpm's as you drive. They really don't make any noise. I am guessing that one of 2 things has happened. 1) it is stuck shut and your exhaust is restricted to the point of choking the engine to death, or 2) the gasket is out between the intake and exhaust manifolds and you are hearing the exhaust leak. My disclaimer....I haven't ever worked on a 6 cylinder and I am thinking that I see a gasket between the 2 manifolds in your picture above. __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

12-21-2003, 02:37 PM	#7
oldsub86 Registered User Join Date: Dec 2002 Location: Canada Posts: 695	It looks like the bolt that holds the two manifolds together is missing. Look at the right side of the photo. There are 2 studs on the exhaust manifold at the back (side next to the engine) that go up through the intake manifold and have nuts on them from the top. There is also a bolt that goes up from the bottom on the front through the exhaust manifold and into the intake manifold. I went through this last summer. The truck started being noisy - exhaust leak. Discovered this bolt was missing and took the manifolds off and separated them. The gasket between was about disintigrated. You don't want to leave it like that as the exhaust will ultimately damage the machined surface between the two manifolds and then you may need a new one. Machining will change the way everything lines up if they have to take much off. Cleaned it up and put it back together with a new bolt and studs. You leave it a bit loose until you have it torqued to the head so that the lineup will be correct. Bolt wouldn't tighten. Had to take it off the truck again and drill out and rethread to a larger bolt size. Touchy as there is just room for the next bigger bolt size. Seems to be ok now. Be careful with the "ears" where the manifolds attach to the head. I broke the front one trying to get it tightened back up. The books advise a torque wrench but about half of the spots won't permit a socket so you have to use a boxend or openend wrench. As to heat riser itself - I wouldn't tie it open or take it out. The spring or the shaft may be worn. If you take the manifolds apart you can check it out. It serves a purpose and wouldn't be there if it was not required. __________________ 1968 Chevy - 292 with a powerglide Last edited by oldsub86; 12-21-2003 at 02:39 PM.

12-21-2003, 08:02 PM	#10
oldsub86 Registered User Join Date: Dec 2002 Location: Canada Posts: 695	I "think" the bolt was 3/8" and I had to go to 7/16". Not sure about length but you should be able to get a good guesstimate by pushing a wire or smaller bolt into the hole and measuring how long it is to the outer edge. ! 1/2" to 2" I "think". __________________ 1968 Chevy - 292 with a powerglide

12-21-2003, 01:20 PM	#4
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	Thanks Stocker, it's good to know what it is and what it's supposed to do. I'm guessing from what you've told me that it's not supposed to flap open & closed at idle? The spring should slowly open it then as the engine warms up? I guess I'll check into prices, if they're even still availble...

12-21-2003, 01:29 PM	#5
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	Da burb, it's definely not stuck shut, that spring may actually be broken or just weak, cause it moves pretty freely for a heat activated spring. The gasket is fine also. It make a kind of rattling sound.

12-21-2003, 02:23 PM	#6
Cat Fuzz My son, my joy. Join Date: Sep 2003 Location: Spokane WA Posts: 463	I had one of those on my small block. Took it out when it broke and extended the pipe to compensate. More of a pain than anything, those are. But your is integrated into the exaust, right? I'd just wire it open to stop the rattling.

12-21-2003, 03:05 PM	#8
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	I don't think I want to wire it open. I'd like the driveability to be there in cold weather. I think the valve leaks around the shaft, so this may be a problem that I can't fix. Oldsub86, I found the place for the 3rd bolt that you said yours was missing. Mine is too. I tighted up the two bolts on the engine side a few weeks ago and thought it was stange that there'd be only two and not at least one on the opposite side of the manifold. Hopefully it just loosened up and fell out. I'll try to figure out what size and length bolt I need - unless you happen to remember. Cast iron gets to be pretty brittle after 30 years, so I'll have to be careful.

12-22-2003, 12:37 AM	#11
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	Well, I got a bolt - 1 1/2" long and the threads in the manifold are stripping. May have already been stripped. I think I'm going to be better off just removing the manifold and tapping the threads and move the next bigger bolt size for the stripped one. It'll have to wait for another sunny day at this point. I got a new gasket to install also. (yawn)

12-22-2003, 11:53 AM	#12
JJR Registered User Join Date: Apr 2000 Location: Leesburg, GA Posts: 746	I had the same bolt problem, it is fairly common. I would develope an exhaust leak from the gasket between the intake and exhaust manifolds. The threads for the small bolt were stripped. You could tighten it a little bit but could not tighten it enough. I ended up installing a heli-coil so I could use the current size bolt. As for the flapper valve, mine is siezed in the closed position. I have an extra exhaust manifold with a good flapper valve but the surface where it meets the intake manifold is warped due to rusting. __________________ FOUNDING MEMBER OF THE DRUM BRAKE CLUB Second Owner 70 C-10 Second Owner 93 GMC C1500 Third Owner 99 C2500 Crew Cab Short Bed

12-27-2003, 10:57 PM	#13
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	Just an update - I ended up removing the heat riser valve completely. It was leaking from the shaft and I couldn't figure anyway to fix this. I was able to remove the pressed in shaft bushing and tapped the hole with a 7/16 x 14 tap. I'll do the other side if it starts leaking, but I suspect it'll be all right. The 3rd bolt on the exhaust manifold that threads into the intake - the one that seems to strip out one everyone - would not respond to any tapping in a possitive manner. After thinking (and praying) about it, I went and drilled straight thru the intake, and stopped as I reached the threads of the stud for the carb. I got a bolt from the hardware store that would work when cut in half (all threads). Looks pretty good at the moment, but I'll know if I forgot anything maybe tomorrow when I put it together. Hope the exhaust flows good without that flap in place. All these small details take way too much time... Attached Images __________________ 1971 K/10 Long Fleetside 350/350 - 6" Lift 1969 C/10 SWB 292 - Sold It

12-27-2003, 11:07 PM	#14
69_hafton Sleep-Deprived Join Date: Sep 2002 Location: Tulsa, Oklahoma Posts: 445	Here's another - of course I had to paint everything when it was apart. Hopefully this info will be of use to others. My intake was very brittle around the threads of that third bolt. Attached Images __________________ 1971 K/10 Long Fleetside 350/350 - 6" Lift 1969 C/10 SWB 292 - Sold It Last edited by 69_hafton; 12-27-2003 at 11:34 PM.

12-28-2003, 12:39 AM	#15
roadking Registered User Join Date: Nov 2003 Location: kc Posts: 229	took mine out this fall. 71 250 same manifolds, gearhead friend of mine says may cause vapor lock in the summer time due to carb over heating ( no heat deflection) if this happens he suggest putting a plate with holes drilled in between manifold to deflect some of the heat. i also bolted the manifolds togeather and had ports milled down at the machine shop, i got a great seal after that with no leaks( very quiet) just make sure you dont crack your exhaust manifold, those babys are very rare. let me know how it turns out.



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