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Old 03-31-2009, 07:33 PM
Xcrement Xcrement is offline
New Member
Join Date: Dec 21, 2008
Location: San Diego
Posts: 2
Newbie Exhaust Question.

What is this gate in my exhaust? It is only on the passenger side behind the headera nd directly above the starter. I doesn't seem to be connected to anything external that actuates it and I can't find it in the haynes or the service manual.

It seems to function properly although it sticks in both open and closed positions. (that is it does not return to open or closed on its own.)
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Old 03-31-2009, 07:39 PM
Chevelleguy's Avatar
Chevelleguy Chevelleguy is offline
Join Date: Jul 20, 2003
Location: Hughes Springs, TX
Posts: 5,565
It is the heat riser valve. When cold, it should close and force exhaust gas through the crossover in the intake manifold to warm the floor of the plenum to aid in cold engine driveability. Most are like yours and not functioning properly and have been welded open or have the butterfly removed so there is no restriction (like mine). It is operated by the bi-metallic spring that is behind the counter weight and manifold heat.
David "If all else fails, read the instructions."
83 Wag Lt,BJ's 6"lift,360/727/Pinned229,D44/trac-lok,AMC20/lock-right,35/12.50 Baja MTZ,Pro-Jection EFI.
10$ NP229 fix

Last edited by Chevelleguy : 03-31-2009 at 07:44 PM.
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Old 03-31-2009, 08:14 PM
Xcrement Xcrement is offline
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Join Date: Dec 21, 2008
Location: San Diego
Posts: 2
Thanks I figured it was something along those lines. This truck has more cold weather equipment than anything I've ever owned.

I will wire it open for now, and remove that butterfly next time the exhaust is off.

Thanks again.
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Old 04-01-2009, 09:44 AM
Jeepguy77's Avatar
Jeepguy77 Jeepguy77 is offline
350 Buick
Join Date: Mar 16, 2008
Location: Corvallis, OR
Posts: 1,183
when I try to drive my truck before the valve opens it sometimes backfires through the exhaust (i can hear it through the passenger floor board), I figure when I go theres quite a pressure buildup and it forces the valve open, so whats the best way to make it stay open, im running an electric choke now on the holley i bought and rebuilt (that was a lot easier than rebuilding the MC 4350) and dont need it to warm the old exhaust heated choke.
sorry for the long post.
77 J10 454, TH400, D20,
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Old 04-01-2009, 10:00 AM
Chris P.'s Avatar
Chris P. Chris P. is offline
Master Mechanic
Join Date: Oct 12, 2003
Location: Mt. Solon, Virginia
Posts: 1,307
You can use a piece of coat hanger wire to hold it open until you get time to fix it right. Like Chevelleguy said, most remove the butterfly and shaft then weld the two holes shut.
79 J10 Honcho 360/T-18/D20
Summit 8600 cam, GM TBI
78 Wagoneer 360/TH400/QT

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Old 04-01-2009, 10:06 AM
GWChris GWChris is offline
Bleedin' Gasoline
Join Date: Jan 22, 2005
Posts: 1,798
It may still be operating - there is a bimetal spring that's not too strong (cannot remember if the spring closes it or opens it - closes it when cold I think). Anyway, it should move freely enough that even when closed the engine is supposed to be able to blow it open.

Finding a good lubricant for it is tough, and actually getting it in there is tougher. I got a real high temp grease from McMaster-Carr years ago, and I try to blow in in with an airhose. I've also tried washing it in with brake cleaner. Mine moves pretty easily.

Some engines rely on exhaust pulses to move exaust through the crossover passage - maybe the firing order has something to do with that.
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Old 04-01-2009, 10:08 AM
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Serious Johnson Serious Johnson is offline
AMC 4 OH! 1
Join Date: May 19, 2002
Location: Pumpkintown, South Carolina
Posts: 3,831
You're probably OK without it most of the time in San Diego, but why not crawl under a couple of times to observe where it is when cold & hot. It may be working just fine. No point in disabling a working system that can do some good and doesn't hurt.

"Carpe Mañana".

'83 Wagoneer
360, .030-over, K8600 cam, Crane springs, ported heads, Edelbrock Performer, G.M. TBI, TFI, 3" exhaust,
T-18a/208, D44/AMC 20 w/ limited slip in both, 3.73s, 33s, BDS 4" springs, Rancho 9000 shocks, etc., etc.
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