Keep blowing rear manifold gasket
Thread Starter
Heel & Toe
Joined: Dec 2006
Posts: 22
Likes: 0
From: Miami Fl
Keep blowing rear manifold gasket
I have an 81 corvette. My problem is that I keep blowing the gasket that sits between the manifold and the block. This is the one at the rear by the oil pressure sensor. I have replaced it twice and it keeps blowing out. I have placed a big ammount of high temp gasket maker to ensure a good seal on the rubber gasket but still it only lasts a few weeks. The previous owner removed all of the pollution stuff from the car. It even has an aftermarket carburator so none of the original piping is there. Can anyone shed some light into this. Is there so much pressure been build in the engine that keeps blowing out that rear gasket...Any help is appreciated
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Is there a PCV valve and breather in use?
On one valve cover there needs to be a PCV valve with a vacuum tube to the carb. On the other valve cover there needs to be a filtered breather.
This system regulates crank case pressure, and I would guess this is why your rear seal keeps going bye bye.
On one valve cover there needs to be a PCV valve with a vacuum tube to the carb. On the other valve cover there needs to be a filtered breather.
This system regulates crank case pressure, and I would guess this is why your rear seal keeps going bye bye.
Thread Starter
Heel & Toe
Joined: Dec 2006
Posts: 22
Likes: 0
From: Miami Fl
Thanks for the quick response. It has a PVC on one of the valve covers and a breather on the other one. The PVC is connected to the air cleaner though and not directly to the carb. Is that a problem?
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
How exactly did you replace the gasket? The method matters, the right way to od it is to use a thin layer of sealer ( I am partial to Ultra Blue RTV) on the gaskets and I sit the manifold in place until it dries, there are nipples on the bottom of the gasket that fit into holes on the block and ribs on the gasket to hold it in place. I then lift the manifold, put on a thin layer of sealant on the side gaskets only and bolt it down. There is a pattern to use for the manifold bolts, similar to head bolts. Start in the center on one side, then center other side, then ends on first side then ends on second side. I usually run all the bolts down finger tight evenly first.
This has worked for every motor I ever did and I think I had 1 blowout. I Use Nothing but Felpro gaskets
This has worked for every motor I ever did and I think I had 1 blowout. I Use Nothing but Felpro gaskets
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
No that's just fine. Now, pull that PCV valve, and shake it. Does it rattle or does it feel blunt and silent?
Edit: I read the above wrong. If the PCV vacuum hose is not hooked up to the carb or manifold then you're not getting proper vacuum.
Last edited by Durango_Boy; Jul 10, 2007 at 08:03 AM.
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
For one thing, you shouldn't use the end gaskets.
All you need is a bead of black permatex about 1/4 inch thick from one head to the other and sealing where the intake gasket on the head touches, and let it dry for about 10 mins then intall intake.
All you need is a bead of black permatex about 1/4 inch thick from one head to the other and sealing where the intake gasket on the head touches, and let it dry for about 10 mins then intall intake.
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
Thats not the way it should be connected. The PCV valve is supposed to be connected to the lower port on front of the carb. It will not get enough vacuume to work correctly on the air cleaner. Move the connection. It may or may not be the problem with your gasket, I doubt it. But, its one more thing to fix
Thread Starter
Heel & Toe
Joined: Dec 2006
Posts: 22
Likes: 0
From: Miami Fl
SixFooter. I installed the gasket just like you described and followed the torque order in the repair book (center first and then out). The PVC valve appears to be fine. At least it functions with the old school way of sucking and blowing on it. I will change the connection to the front of the carb. Let see if this fixes it.
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Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
It is best as Tim said to not use the end gaskets, but be sure to clean all surfaces with lacquer thinner first to get rid of all the oil etc. Preferrably let the rtv set up at least overnite before starting the engine.
If you don't have the 3/8" port on your carb, you can use intake vacuum but preferrably not on a single runner. You may tee into the power brake fitting if no other.
Depending on the blowby on your engine, it could blow out all seals and make the motor leak like hell. It can even blow out your dipstick.
If you don't have the 3/8" port on your carb, you can use intake vacuum but preferrably not on a single runner. You may tee into the power brake fitting if no other.
Depending on the blowby on your engine, it could blow out all seals and make the motor leak like hell. It can even blow out your dipstick.
Instructor
Joined: Apr 2007
Posts: 184
Likes: 1
From: Atlas is shrugging KC, MO area
SixFooter. I installed the gasket just like you described and followed the torque order in the repair book (center first and then out). The PVC valve appears to be fine. At least it functions with the old school way of sucking and blowing on it. I will change the connection to the front of the carb. Let see if this fixes it.
Racer
Joined: Oct 2005
Posts: 469
Likes: 0
From: West Slope of the Sierras California
Thats not the way it should be connected. The PCV valve is supposed to be connected to the lower port on front of the carb. It will not get enough vacuume to work correctly on the air cleaner. Move the connection. It may or may not be the problem with your gasket, I doubt it. But, its one more thing to fix
I was having that same problem with the PCV valve plumbed to the base of the air cleaner. Moved it to the correct port on the carb (per Holleys instructions) and no more blown seals.
Race Director
Joined: Jun 2000
Posts: 10,046
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From: FL
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
WRONG! 
Melting Slicks
Joined: Jul 2004
Posts: 2,577
Likes: 2
From: Long Island New York
Just make sure that both surfaces are clean and free of any old gasket particles or old sealant. Also make sure to put a new gasket back on the distributor shaft as well, this another place where you can get an oil leak if you don't.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,392
From: At my Bar drinking and wrenching in Lafayette Colorado
I'll just summarize and confirm what the guys have been saying:
First, your PCV being hooked up to the air cleaner is a big problem. This is allowing the crankcase to pressurize slightly and induce the blown seal problem. The PCV must be hooked up to the direct manifold vacuum fitting at the base of the carb so that the PCV is pulling a slight vacuum on the crankcase.
Second, you need to delete the manifold end seals - do not use them. They are notorious for leaks and blow-out. Use a 1/4" - 3/8" bead of silicone sealant, such as "Ultra Black", in place of the seals. This will eliminate any blowout possibility.
Lars
First, your PCV being hooked up to the air cleaner is a big problem. This is allowing the crankcase to pressurize slightly and induce the blown seal problem. The PCV must be hooked up to the direct manifold vacuum fitting at the base of the carb so that the PCV is pulling a slight vacuum on the crankcase.
Second, you need to delete the manifold end seals - do not use them. They are notorious for leaks and blow-out. Use a 1/4" - 3/8" bead of silicone sealant, such as "Ultra Black", in place of the seals. This will eliminate any blowout possibility.
Lars
Le Mans Master
Joined: Oct 2001
Posts: 5,943
Likes: 3
From: Wildwood IL
To ensure a blow proof silicone seal, I always use a center punch and dimple the block & intake a bit so the silicone has something to bite on. Eddie
Instructor
Joined: Jan 2007
Posts: 167
Likes: 0
From: Provo Utah
Theseguys all know what they are talking about, but when i replaced mine i used the above instructions and RTV black did not work for me (could have been human error). I bought some stuff called "The Right Stuff" and used it. No waiting needed, just bolt it down and go. since then I have not found a drip of oil. Do these steps, but maybe just for extra comfort give the Right Stuff a try. Its about $10 for a can if i remember right.
Melting Slicks
Joined: Feb 2003
Posts: 3,053
Likes: 4
St. Jude Donor '03,'11
just to reiterate what has been said several times.
don't use the front and back rubber gaskets. When you lube them up with silicone they just squeeze out. If you put them in dry they squeeze out or blow out.
And if the heads have been shaved even 10 thousandths the gap is smaller causing the problem to be even worse. Mine were popping out even before i started it.
run a thick bead of silicone only. That works.
don't use the front and back rubber gaskets. When you lube them up with silicone they just squeeze out. If you put them in dry they squeeze out or blow out.
And if the heads have been shaved even 10 thousandths the gap is smaller causing the problem to be even worse. Mine were popping out even before i started it.
run a thick bead of silicone only. That works.
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
just to reiterate what has been said several times.
don't use the front and back rubber gaskets. When you lube them up with silicone they just squeeze out. If you put them in dry they squeeze out or blow out.
And if the heads have been shaved even 10 thousandths the gap is smaller causing the problem to be even worse. Mine were popping out even before i started it.
run a thick bead of silicone only. That works.
don't use the front and back rubber gaskets. When you lube them up with silicone they just squeeze out. If you put them in dry they squeeze out or blow out.
And if the heads have been shaved even 10 thousandths the gap is smaller causing the problem to be even worse. Mine were popping out even before i started it.
run a thick bead of silicone only. That works.
Le Mans Master
Joined: Oct 2004
Posts: 5,269
Likes: 54
From: Orlando Florida
St. Jude Donor '05
Thanks Everybody For Posting This !!!
I will now go and see if I have a vacuum connection available at the carb..... I've been running the engine with the PCV connected to the air filter
I will now go and see if I have a vacuum connection available at the carb..... I've been running the engine with the PCV connected to the air filter