5.7 manifold errosion
 

Just pulled the manifolds and risers off my 5.7. Engine was installed in 1998, almost unused till I bought it in 2009. I replaced the risers then, but the manifolds looked almost new, so I did not remove them.

4 seasons later is now. Manifolds still look ok to me, but the seat on the block is erroded around the bottom of the exhaust for the center two cylinders on both sides, see pics. The erosion is about half way across the sealing surface. I don't remember seeing this on previous engines.

Is this normal? If not, do I need to to more that just bolting a new set of manifolds on? I could clean it up real good and marinetex it and sand smooth, but that's a lot of work. Ideas?

You should replace the manifolds and risers every 2 to three years. You are asking for trouble by not replacing them. You cannot see the passage inside and could be rotted. If this rotts out while you are on the water or even out of the water with water in the manifolds it will end up going in your motor (cylinders) and will hydro lock, and you will have even bigger problems.

New manifolds and risers are bought and ready. The old ones look amazingly good, but I'd prefer not to gamble.

Erosion on the block is what is worrying me.

looking at the picture:

see the small rust blobs ??

last time you did a compression test ?

Side question here...is there any difference between OEM manifolds and the "other" type that are always less expensive?

If so what are the top 2? brands?

Good question. That's one of the reasons I left the Volvo manifolds on there when I bought the boat and replaced the risers. This time, both old risers and old manifold are Volvo OEM. The manifolds I've got to replace them are HGE Marine Engine Products, Chinese made, bought through Jerrys Marine. Looks like a bit thicker metal, but I'm a little worried,thought I was buying American made. New risers are Volvo. Looks like everything should fit together fine.

Compression test just before pulling manifolds, 186 to 200 all the way around. Suspect my meter is reading high, but its reasonably even. not sure what you mean by rust blobs?

the metal is thin there in the middle I would throw some high temp sealant on the gasket and go fishing.

Connor, what's going on there is pretty much a classic case of what we used to call oxidation/erosion in the gas turbine business. The metal gets hot (I'd guess about 1200F in this case), oxidizes (rust in the case of iron) and then the brittle oxide spalls off or erodes away when the metal expands and contracts during throttle cycles. Used to see it all the time on turbine vanes and segmented outer air seals. Mechanism is the same regardless of alloy. Pratt & Whitney has some fancy plasma-sprayed ceramic coatings that would fix that, but it would cost more than the cylinder heads! Don't bother with Marine Tex . . . I suspect it will just start turning to carbon at about 600F!

One thing that I'd look at is how the exhaust gasket and manifold fit over the port. If there is a mismatch where a portion of the exhaust gasket and/or manifold sticks up in the "breeze" at the bottom of the port and blocks the flow, you'll get "stagnation" conditions where the flow comes to a stop at the obstruction and that surface can be quite a bit hotter than one with gas flowing smoothly over it. How much hotter it would get depends on the velocity (Mach Number) of the gas . . . at Mach 0.5, it would only be about 5%, but at Mach 1.0, it would be 20%! Anything you can do to smooth out the flow going into the exhaust manifold would reduce the temperature in that area.

I'd say the main concern is reduced wall thickness in that area where it's eroding and how far it is from the water jacket. If you had access to a scrap head that you could cut up, that would give you some idea of how much margin you have. Maybe Pelican or some other guys on the forum have cut up a head or have run one like this to failure and could give you a better idea of the ultimate failure mode. I suspect it would crack well before it actually eroded into the water jacket. If it 's a FWC engine, loss of coolant would be easy to detect, but if it's raw water cooled, a leak there would quickly kill the engine. If it just eroded through at the gasket surface to create an exhaust gas leak to atmosphere, that would be easy to spot . . . what us old turbine guys would call a "graceful" failure mode! Denny

Yeah what he said. :)

Thanks all.

I suspected a mismatch involving the gasket or in how it sealed. The manifold side is perfect, the problem only on the engine side and on both sides of the engine. The old gaskets did not separate the two center exhaust holes, which I thought was strange. I'll check tomorrow and try to see how the old gaskets went on and how the new ones will. Old ones are some kind of tough, fiber mat encased material. New ones are more like paper. Maybe I can trim the gasket to fit better around the eroded space.


Engine is fresh water cooled. I had envisioned the possibility of eroding through and allowing hot gas to escape to the outside. Never occurred to me that it could erode or crack into the water jacket. Ugh!

Hey Connor,

The exhaust manifolds are obviously running a lot cooler than the heads if there is no oxidation on them. Since the engine is fresh water cooled, the risk of keeping those heads is minimal and I think I'd just put the new manifolds on it and press on, but keep a close eye on coolant level. If you see it start to drop, loosen the Hex cap to eliminate pressure in the system that would force coolant out thru any cracks. The gasket surface is at the extreme outer edge of the cylinder head casting, so I'm guessing the head is fairly thick in that area. If you can trim the gaskets to match the ports and eliminate any flow restrictions at the head/manifold joint, that would help. I'd trim the gaskets to match the ports on the head, and then see how they match up to the exhaust manifold. The old gasket might have been some sort of asbestos material that's no longer available, but carbon fiber would be an excellent material if you could find some, as it will probably withstand about 1900F! If the manifold is smaller than the trimmed gasket, then I'd break out a grinder and open up the manifold to match the gasket. Maybe use some of that muffler repair putty as a gasket sealant. It's basically a ceramic material and will take some fairly high temperature. Those siamesed exhaust ports are the hottest spots on a small block Chevy and are one reason the Fords tend to be a bit more durable in that area since they have individual exhaust ports. The big block Chevy and Chrysler Hemi also have individual exhaust ports to minimize hot spots. Interesting that they didn't even try to seal the area between the ports, but I think I've seen automotive exhaust manifolds with just one big opening for the center ports. That would explain why you've lost material from the rib between the ports, but that's no big deal. I'm guessing you didn't see any problems on the end ports.

Although I've heard that you generally can't justify the extra cost of FWC in a gas engine, this is one situation where it allows you to keep running a set of cylinder heads where it would be a lot riskier in a raw water cooled engine! Your valve seats are obviously in good shape with those compression numbers, so good thing you were observing the 7" limit on manifold vacuum! It's strange that the exit of the exhaust port is evidently running hotter than the exhaust valve itself or the valve seat area, although maybe Volvo uses better materials in those areas! Denny

A few thoughts.

A 99 engine will be tuned for non ethanol gasoline. I am not sure if this is a MPI or a carburetted engine, but either way, it will be running lean. Lean and hot probably contributes to this as there is excess oxygen at the siamesed ports. More so than a properly tuned system.

If you can re-jet or change program or injectors, that isn't a bad idea for this port and the exhaust valves. A little bit rich wouldn't hurt. If it has carbs and you can just go up a bit on the main jets, that might be the ticket. Without excess oxygen in the exhaust, that erosion and oxidation will slow down considerably.

And I am with Bushwacker on the muffler putty.

If it's an EFI engine, just buy another temperature sensor, mount it somewhere cool and connect the wire from the engine computer to that one instead of the one on the engine. This will fake out the computer and make it think the engine is cold so it needs a richer mixture to adequately vaporize the fuel. Might hurt your gas mileage a bit, but should generate a little more power, plus it's probably cheaper to burn fuel than cast iron, valves and valve seats!

Fishstretcher. I run exclusively what is supposed to be non alcohol gas in this thing. Caveat: I've noticed over the last couple of years that there is no water at all in my fuel filters. I suspect there is some alcohol in what I'm buying.

Bushwacker., Its an early EFI, throttle body system. Sensor in a cooler place is a good idea. I'll explore how to do that with my mechanic.


Raining today, it may be a while before I can get back to the boat.

Connor, you could rig up the sensors with a switch to select either one, sort of like an economy mode using the OEM sensor for lightly loaded operation and then a power option with the other sensor for heavy load and/or hard running. Denny

If you are running non ethanol gas, then you are already taking pretty good care of it. I hear you can reprogram the 555 based systems. I don't know if that includes the TBI. The colder sensor or resistor in the wiring harness is a trick on older systems like that.

I suspect this would help. Maybe a fair amount. But at some point it is just cast iron in a condensing marine environment, so there will be a finite life. Which means I should look at mine, too.

edit: I wonder if edelbrock etec 170 heads would fix this. They are aluminum and supposed to be a replacement for the vortec heads. Something I am interested in, but haven't fully researched.

Aluminum heads might actually work very well for a couple of reasons. It's improved conductivity relative to iron makes it easier to cool down hot spots. And although it has a much lower melting point than iron, the aluminum oxide that forms at high temperatures is actually a tough ceramic that tolerates very high temperatures in addition to acting as insulation. Although turbine vane alloys are nickle based, the better ones contain a significant amount of aluminum, which forms a protective oxide on the surface. The oxide will eventually spall off, exposing fresh material which then forms more aluminum oxide on the surface, so it's a self-healing process. We used to enhance it by just packing the parts in a retort full of pure aluminum powder and then cooking them at high temperature in a vacuum furnace. The aluminum would diffuse into the outer 5-10 mils of the surface and then create a very robust aluminum oxide layer when run in an engine. The same thing will happen in the exhaust ports of an aluminum head!

This oxide formation is one reason why the V-4/V-6 outboards can get away with dumping hot exhaust gas into the V of the aluminum block, but it also forms on bolt hole threads, which is why you often had to use a propane or acetylene wrench to remove the bolts from the exhaust covers on the old cross-flow motors! I'd be sure to use plenty of Never-seize on the exhaust manifold bolts!

FWIW/FYI: In twenty five years of high volume outboard service experience with aluminum engines, we preferred to stay away from "never seize/ anti seize" compounds because of galvanic corrosion coupling between the aluminum/ metallic never seize anti seize compound/ stainless components. We maintained and serviced the same client's boats for years and years, and found that a Teflon type anti corrosion grease seemed to work best at isolating the different metals, and preventing seizing and corrosion.

So does that mean tef-gel versus nickel or copper anti-sieze? I figure nickel and copper are more noble than aluminum (without looking t up).

Bill, thanks for the heads up. I'm aware of that issue, but most folks don't know that there are metals as well as graphite in never seize. I don't doubt that it could cause a problem in aluminum and stay away from it for that purpose. I used to use it on my volvo props, which are famous for sticking to the shaft. Never had a problem, but I use anti corrosion grease now.

That's good advice, as the graphite is highly conductive and could increase galvanic action between aluminum and whatever more noble materials are in contact with it. Frank Brown, who was a well known Merc mechanic that used to crew for Carl Moesly on the race boats, used a mixture of anti-corrosion grease and Never-Seize on prop shafts and I've had good luck with that, but the mating parts are steel and brass/bronze, so no aluminum is involved. I normally use anti-corrosion grease on bolts, but Tef-gel would be a good choice for SS & aluminum combinations.

tef gel is a new one on me. What's that?

Teflon in a white gel form. Really sticky and gooey. I usually get more on me that what I am working on anyway. This should surprise everyone. Makes a nice barrier between dissimilar metals. Used it on my LP @ 7 years ago and no corrosion between ss and aluminum.

I recently learned about its conductivity properties. You can use it in butt connectors and such and add virtually no resistance. So I was told anyway.

Cheers,
GFS

Well, I think its ok, I think. I've got a very long trip planned for next summer and this kind of "its ok, I think" can keep me up at night.

Pic below shows that the gasket doesn't cover the entire face of the head. The erosion was on the part of the head exposed and it appeared to stop once it got down to the gasket. I checked old and new gaskets, they cover an identical area, The outer two cylinder's exhaust show no issues, but I guess they run a bit cooler.

I think I will pull the manifolds next winter, just to check. . . . Maybe just before next summers trip would be a good idea. What can I say, I'm paranoid.

Anybody seen something like this before?


Opps, only took one pic and thought it was in focus, sorry. You can still mostly see that the gasket does not cover the area that was eroding.

Judging by an image search, at the port-manifold face there is a lot of metal. Further in the port, say 1/2" or more towards the valve, the water jacket comes into play.


The two links below show how far you can and can't go. Although both seem to be seriously over ported to me. Gasket matching isn't a good basis for a template.

To me, your head looks ok. I think if it were a problem, then it would be a problem common to all marine vortec head equipped small block chevys. (I am assuming you have a vortec head. The internet seems to think so, too.)


http://www.pacificp.com/forum/viewto...c2a63762107ee9

http://www.justinb.net/bulletproof/page4.html

Thanks Fishstretcher, just what I needed. Looks like I'm ok. I will pull the heads next year and look again, maybe grind the rough areas smooth, but not try to match the gaskets. Exhaust flow reversion is a buggabo that I worried about with the engine so deep in the hull.