5.7 Cast Iron Intake Manifold - Salt (‘raw’) Water Cooled

[Brentathon]

How long do these cast iron manifolds last?….
Geez.
This is saltwater cooled since 1991.
I guess ya just run them until they fail….?

 

[Bondo]

How long do these cast iron manifolds last?….
Geez.
This is saltwater cooled since 1991.
I guess ya just run them until they fail…

Ayuh,..... 5, maybe 10 years life span, they usually fail at the manifold to riser joint,.....
If you run 'em til they fail, odds are you'll be replacing the motor at the same time,....

 

[tank1949]

Pitch the motor!

 

[Lou C]

If you're referring only to the intake, I have one on my 4.3 which has been on it the whole time I've owned it in salt water in my use since 2002. Not flushed during the season ever, it is a moored boat so full of salt water from May-November every year. Still hasn't rotted through yet. But I do flush at the end of each season, drain and then back fill with a good PG antifreeze with corrosion inhibitors. I pull the stat housing and check the area right under the stat, that is where they fail if the cast iron gets weak there. Mine, is still solid but I do have a spare 4.3 intake in the garage.
If you're referring to exhaust manifolds and elbows, well, 7 years is about it.
Tap that area under the thermostat with a big screwdriver, if it's solid and no water in the oil, just run it.
From what I can see that area doesn't look bad but the rest of the engine looks like it's been having salt water sprayed on it from a leaky hose. Very important check all hose clamps and drain plugs make sure there are no raw water leaks.
I have heard of some engines rotting through in the water passage behind the exhaust valves and hydrolocking the motor, 1991 was a LONG time ago, if that engine doesn't have water in the oil that's gotta be some kind of record.

 

[Brentathon]

Ayuh,..... 5, maybe 10 years life span, they usually fail at the manifold to riser joint,.....
If you run 'em til they fail, odds are you'll be replacing the motor at the same time,....

5 - 10 years??? I’ve owned this ‘91 boat since 1999, and that’s the original intake manifold. I also know someone who owned a 1991 wellctaft with the same engine, and there’s wasn’t rotted out, yet, in 2019 when they got rid of it.

 

[Brentathon]

Pitch the motor!

If you're referring only to the intake, I have one on my 4.3 which has been on it the whole time I've owned it in salt water in my use since 2002. Not flushed during the season ever, it is a moored boat so full of salt water from May-November every year. Still hasn't rotted through yet. But I do flush at the end of each season, drain and then back fill with a good PG antifreeze with corrosion inhibitors. I pull the stat housing and check the area right under the stat, that is where they fail if the cast iron gets weak there. Mine, is still solid but I do have a spare 4.3 intake in the garage.
If you're referring to exhaust manifolds and elbows, well, 7 years is about it.
Tap that area under the thermostat with a big screwdriver, if it's solid and no water in the oil, just run it.
From what I can see that area doesn't look bad but the rest of the engine looks like it's been having salt water sprayed on it from a leaky hose. Very important check all hose clamps and drain plugs make sure there are no raw water leaks.
I have heard of some engines rotting through in the water passage behind the exhaust valves and hydrolocking the motor, 1991 was a LONG time ago, if that engine doesn't have water in the oil that's gotta be some kind of record.

Yep… it’s a 1991; the long block was replaced in 2007 with a rebuilt, and that’s the original intake manifold. I don’t think they make this iron manifold anymore… “pre-vortec”, and I only have 1 inch clearance under the engine cover (i.e. deck), so I’m not sure if the vortec is a “high rise”, and whether it will fit.
But… I guess I’ll keep running it as long as it lasts.
These Chevy 350’s are amazing, how long they can last with saltwater running through them.

 

[Brentathon]

Ayuh,..... 5, maybe 10 years life span, they usually fail at the manifold to riser joint,.....
If you run 'em til they fail, odds are you'll be replacing the motor at the same time,....

Maybe you were referring to exhaust manifolds. I was referencing intake manifolds.

 

[Lou C]

You have to use the intake that matches your head design; pre Vortec heads require the pre Vortec intake. 12 bolts instead of the 8 bolts on the Vortec and the pre Vortec used the exhaust crossover.
They may not be made new but can be easily found used. I found a few good ones for my 4.3 pre Vortec but the original as I said is still fine. Check on eBay…

 

[Scott Danforth]

the cast intake manifold will last just as long as the head castings and the block casting in salt water.

right up to the point where one of them rusts thru to the crank case. the intake will rust thru near the water cross-over outlets
heads will rust thru near the exhaust valves
block will rust thru near top of cylinder walls

 

[Brentathon]

the cast intake manifold will last just as long as the head castings and the block casting in salt water.

right up to the point where one of them rusts thru to the crank case. the intake will rust thru near the water cross-over outlets
heads will rust thru near the exhaust valves
block will rust thru near top of cylinder walls

So…. I’m wondering if I should bother to replace the intake manifold…because it’s original since 1991.
Can that be done with motor remaining in boat…?
The block and heads were likely decent enough condition in 2007, when I got the rebuilt long block.

 

[Brentathon]

Well… I just spoke to my mechanic that I’ve used off/on for the last 28 years…
He said he thinks he’s only changed one cast iron intake manifold for rusting through…and thinks I can continue to run this thing.
Interesting…

 

[Lou C]

It's a bit of a gamble but you can poke at the intake right under where the thermostat sits and see if it's getting weak there with a lot of peeling flaking rust. I take a small hammer and a screwdriver and tap at it to see if it sounds solid. And, while you'll get a lot of debate on this topic, my experience with filling the engine with good PG antifreeze with corrosion inhibitors for winter storage appears to have help it last longer, maybe....my thought being we spray anything on the trailer that we don't want rusting with Corrosion X/Fluid Film/Boeshield, why not do something similar to the inside of the engine?
BTW Merc themselves has recommended that for some time now.

When I changed the heads on mine in 2017 (previous overheat damage) they had been used in salt water for at least 17 seasons at that point and had not rusted through and when I had the intake off I could thoroughly inspect it and it looked good all thru that water passage. The other thing you can do is check the oil frequently and as soon as you see any trace of water investigate.
Yes you can change the intake with the engine in the boat, I did the whole top end over haul with the engine in the boat. Not sure why but the old intake gaskets on mine were like kryptonite and it was very hard to get off. Maybe the overheat baked them on, who knows.....

 

[cyclops222]

Antifreeze fluid should stop all the rusting during the winter. Right ?
No standing Oxygen and salts.

 

[Scott Danforth]

Yes you can change the intake in the boat

New in 2007 was 18 years ago. That is longer than the boat was designed to live

 

[Lou C]

What I’ve seen around here looking at a number of I/O boats for sale is that very few have closed cooling. Nearly all are raw water cooled yet most seem to have at least a 10-15 year lifespan if not longer as long as the exhaust which deteriorates faster is replaced on time. Ive said many times if I repower this boat with a reman or new engine that’s a must for me, as well as a remote oil filter mount…
For now the old 1988 4.3 just keeps going with the original short block intake & reman heads installed 8 years ago. When I see water in the oil it will be time….

 

[Scott06]

Yep… it’s a 1991; the long block was replaced in 2007 with a rebuilt, and that’s the original intake manifold. I don’t think they make this iron manifold anymore… “pre-vortec”, and I only have 1 inch clearance under the engine cover (i.e. deck), so I’m not sure if the vortec is a “high rise”, and whether it will fit.
But… I guess I’ll keep running it as long as it lasts.
These Chevy 350’s are amazing, how long they can last with saltwater running through them.

you can likely find a used fresh water one the dimesnsion listed for the marine edlebrock probably doesnt fit what you outline for height

Edelbrock Marine SBC - V8 Performer Intake Manifold W/BRASS (1987-95)

michiganmotorz.com

 

[cyclops222]

As my ,Marine Mechanic keeps telling me.
If it is not broken. Do not fix it.

 

[Lou C]

If you're concerned you can always remove the intake and check it more thoroughly by looking at the water ports, often you start to get some pitting there, but if you use a bit of Permatex Aviation in those spots when you install the gasket, it will seal, at least in my experience.

 

[tank1949]

Maybe you were referring to exhaust manifolds. I was referencing intake manifolds.

Having owned several I/Os over decades that were all saltwater used, rigorously fresh water flushing after every trip will increase block mortality to over ten years and maybe 4-6 years for risers and exhaust manifold. The risers are the first ones to crap out. I'd replace every 4 years and sleep well.

 

[ROY WILLIAMS]

How long do these cast iron manifolds last?….
Geez.
This is saltwater cooled since 1991.
I guess ya just run them until they fail….?View attachment 406225

damaged engine rusted !!!!!!!!!!!! engine damaged !
new engine .