Hole in ex. manifold (pics)

[350kmileford]

Not the standard hot air tube. This is on the bottom of the manifold, close to the engine side. I noticed the leak about a month ago, appeared when I started the car after it'd been sitting several days:

http://i44.tinypic.com/2psktup.jpg
http://i41.tinypic.com/e128o4.jpg
http://i39.tinypic.com/hsta3s.jpg

well? (At least it's not as bad as the hole in my last manifold http://i44.tinypic.com/24pzaiv.jpg)

 

[JackFish]

Isn't that the vent hole for the pipe that's supposed to go up to the carb?

What year is that manifold?

 

[350kmileford]

Is it? Cause there's also the (other) hot air hole that's higher up on the manifold, and also more easily accessible.
Block/manifold should be about a '66

 

[JackFish]

On mine there are two holes, the one on top goes to the choke, and the one on the bottom goes to a port on the carb venturi. The holes are not getting exhaust gas, there's a chamber in there that just supplies heat.

 

[350kmileford]

The one in the pictures was producing hot, pulsing air. And the hole is (obviously?) cast into the manifold. Hmm, maybe the sealed metal broke?

 

[fordconvert]

That is just supposed to be a tube that goes through the exhaust manifold. The vacuum comes from the carb and gets sucked through the long tube and then is heated as it passes through the exhaust manifold. Its not supposed to get any exhaust gas. That one bolt looks loose. My guess is you got a slight leak there and are running a bit rich. With the leak a bit of fresh air gets drawn in and if there is still unburnt fuel it burns and turns the manifold into brittle mush.

Here are some photos of mine. It does not look that bad, just those little tiny cracks right?


Now look at the other side!

 

[350kmileford]

That is just supposed to be a tube that goes through the exhaust manifold. The vacuum comes from the carb and gets sucked through the long tube and then is heated as it passes through the exhaust manifold. Its not supposed to get any exhaust gas. That one bolt looks loose. My guess is you got a slight leak there and are running a bit rich. With the leak a bit of fresh air gets drawn in and if there is still unburnt fuel it burns and turns the manifold into brittle mush.

You think I have a leak cause of a loose bolt or I just have a leak at the hole? I replaced the gasket a week or so ago and all bolts are tight. If there was any sort of exhaust leak why would fresh air be getting drawn in? Also, my car's always ran a bit lean.... Either way, it gets extremely hot on the freeway and I've seen the ex. manifold glow orange after drives before. Reckin I'll remove the manifold tomorrow and inspect the inside. When I pulled it a week or so ago I looked inside a bit but didn't do a thorough inspection.

Edit: That bolt does look loose in the picture, I will check. It was tight a few days ago when I checked them. The noise hasn't changed any since though.

 

[fordconvert]

Because the exhaust flow is pulses you get situations between the pulses where its scavenging or sucking. Think of the tube system at the bank drive up window. Your exhaust is like the tube. Each exhaust pulse is like the capsule. As it goes through the pipe its pushing the air in front of it which would push what ever is in front of it along. Its also sucking air behind it as it goes. This air has to come from somewhere. Most of the time this works out good because after the pulse comes out of the cylinder that little vacuum pulse is behind it scavenging the little bits that are left. If you have a bad gasket or crack and one of those little low pressure zones between pulses goes past it the outside (high) pressure air will get drawn in. If there is unburnt fuel there (from running rich) it can continue to burn in the exhaust manifold. Thats basically what the smog pumps did. They pumped fresh air into the exhaust stream to help it finish burning. Once cats came in on some models they smog pump pumped fresh air into the cat .

What happens on our stock 6's is the manifold cracks near the center just due to the stress of the rest of the system hanging off it. This crack is often near the inlet for that choke tube. The choke begins to suck exhaust. The carbon and heat from the exhaust begins to keep the choke from opening all the way which makes the engine start to run rich. Rich running sends lots of un burnt fuel into the exhaust. That fuel combines with the air from the crack and burns hotter. At this point the crack begins to get bigger which makes it burn hotter. Hotter then burns out the gasket which leaks more which lets it burn hotter. Most likely by that time the choke is mostly stuck shut and running really rich. You then get a manifold that looks like the one in my picture. You also end up needing a drill bit to get the carbon out of the choke vacuum passage in the carb. That choke tube in the photo you could pick pieces out of it with your bare fingers. Not sure what sort of temps it takes for cast iron to get that brittle brittle but it can obviously happen. I bought the car not running so I have no idea how long it took for all that to happen. I did run it for a while with that manifold and you could tell it leaked a but it was not that bad. I was actually thinking of not replacing it at first. I only decided to replace it because I decided to replace the rest of the exhaust system and figured I might as well just replace it all.

 

[tony1963]

An inexpensive fix for this would be to grind that crap out of the manifold, then insert a piece of black iron pipe between the sections where you ground away the casting. THEN, insert a steel line through the manifold to get your hot air choke working again.

Older Chevrolets used the same type of hot air tube choke. However, theirs was not insulted with cast iron.

Sliding the black iron pipe in place would shield the tube and provide some rigidity to the manifold.