Port motor down...choices choices choices! What did you choose?

Fascinating pics, thanks for sharing! Are your exhaust flappers/shutter valves still intact and or in good shape? Maybe you had a small leak around the exterior of the flappers so water backed up with the continuous river pressure.
Straight inboards, so no flappers or shutter valves, just a water lift muffler that would normally let exhaust flow out without water backing up into the exhaust. But I'm thinking it was a "perfect storm" of conditions and operator error--I was backing upriver against a strong current. The port side was also against the dock floats, creating even more back pressure. With the exhaust opening underwater, all that extra pressure found somewhere to go, and doing it for about 15 seconds under those conditions ws more than enough to overwhelm the water lift muffler's capacity. And just being real, I think I made an error that was probably the final nail in the coffin :rolleyes: Since I was focusing on positioning the boat and shutting down to refuel, I probably turned off the engines immediately without letting the mufflers, exhaust, and intake "clear" from the valve overlap....meaning as soon as I went to restart, I probably sucked just enough water backwards into the cylinder through the exhaust valves to do the damage.
 
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Strange indeed. Can you get a close up pic of the severed fire ring? How would compressed water take out such a large piece?
 
Strange indeed. Can you get a close up pic of the severed fire ring? How would compressed water take out such a large piece?

Sure thing, I'll get more pictures when I'm down there in the next couple of days. But if there were water sucked into a cylinder by the valve overlap on the exhaust/intake strokes as an engine was shut off, it would then have to displace somewhere on the next compression stroke--valves are closed and the piston is coming up. The "path of least resistance" would seem to be deforming/severing the thinnest point of the head gasket between cylinders rather than breaking metal in the valvetrain, especially a head gasket with 975 hours. It doesn't even have to break the gasket (although enough water volume might), just deform it enough that it's weak where more combustion and compression will finish the job as the engine continues to run.
 
You have the same exhaust I have......what condition are the exhaust manifolds and risers in?

Also from the picture the head gaskets are blown in a direction toward the front of the motor. I have worked on gas engines (race cars, boats, trucks etc.) for almost 50 years and never seen anything like this.

Inexpensive fix given what could have been wrong.
 
You have the same exhaust I have......what condition are the exhaust manifolds and risers in?

Also from the picture the head gaskets are blown in a direction toward the front of the motor. I have worked on gas engines (race cars, boats, trucks etc.) for almost 50 years and never seen anything like this.

Inexpensive fix given what could have been wrong.

DEFINITELY could've been way worse! I'll take a blown head gasket (or two) over broken top or bottom parts any day LOL

The manifolds and risers--the boat has been local freshwater its entire life. The original owner had the boat till 2012, spent stupid money keeping it in showroom condition (things like new batteries every year) and kept meticulous maintenance records--but no mention of ever replacing them. Last two owners kept no records at all, but I did talk to the PO's mechanic before purchase, he said he did not do them since buying it in 2016. The risers are still Mercruiser stainless steel, and Mercruiser quit producing those back around 2000....so I'm thinking mine are still the originals. They look to be in decent shape--just a tiny bit of rust at the gasket, nothing bad and no leaks. No rust or discoloration in the gas chambers, just healthy black...nothing but superficial discoloration in the raw water openings, no discoloration in the closed cooling openings. But being suspected 26 years old, I'm replacing them anyway.

Two things on the heads have me a little concerned...first, every cylinder has a deformed jug opening--as if maybe there's been excessive heat or uneven burn over the years? The jug holes in the gaskets aren't nice and round, every hole has a slight deformity. Also, at the top of the #7 bore (the one next to the blue rag with the most coolant in it) I notice what appears to be maybe a bore deformity at the very top edge, with what looks like a rust discoloration? I didn't notice it as I was pulling the heads, just in the pictures. Anyway, I'm sure these are also original parts. Hopefully the new top end will remedy any past issues of water intrusion or overheating, and the bottom end seems well lubricated and still very "tight" so fingers crossed I'm good. I'll post more detailed pictures when I'm back down at the marina.

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Many times the simplest answer is the correct one ..
Heard those stainless elbows with the standard cast manifolds are known to leak. Could be a reason they stopped making them years ago.. thinking you just injested water from an old fashioned elbow to manifold leak.
Most common Mercruiser engine failure point.
River situation could just be a coincidence...?
Parts are available in either ceramic coated or E-coated.
I like the ceramic coated and I’ve never had a failure but, read the E-coated are just as good and will save you money.
Make sure to use the silver OEM elbow gasket!
https://nuwavemarine.com/7-4-454-502-exhaust-parts/

May also want to change the elbows and gaskets on the other engine at this time to save the same occurrence from happening..

Best of luck keep us updated ,

M
 
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Many times the simplest answer is the correct one ..
Heard those stainless elbows with the standard cast manifolds are known to leak. Could be a reason they stopped making them years ago.. thinking you just injested water from an old fashioned elbow to manifold leak.
Most common Mercruiser engine failure point.
River situation could just be a coincidence...?
Parts are available in either ceramic coated or E-coated.
I like the ceramic coated and I’ve never had a failure but, read the E-coated are just as good and will save you money.
Make sure to use the silver OEM elbow gasket!
https://nuwavemarine.com/7-4-454-502-exhaust-parts/

May also want to change the elbows and gaskets on the other engine at this time to save the same occurrence from happening..

Best of luck keep us updated ,

M
Thanks for the tips! I have actually been shopping for and getting ready to do an order online for risers and manifolds for both engines, but didn't realize there were different options available other than just plain cast iron :) The only reason I hadn't ordered yet is because I notice when you shop for risers that there are three different gasket choices, and I'm not sure which one we use. There are full open and half open for our wet exhausts, then full restricted for dry exhaust (that we don't use). Do you remember if our manifold to riser gaskets have all four slots open, or have two open and two with the small holes? I was going to take one apart this afternoon to see, but you could save me some time LOL

On the root cause...I decided to not overthink it. I know it was water intrusion back through the exhaust valves, and the manifolds/risers are due for and getting replacement either way. I lean away from the known leaky exhaust issue just because it happened to both head banks simultaneously as I was docking for fuel, not just one side or the other. For both banks to have a simultaneous part failure on an engine that was running perfect, now THAT would be a crazy coincidence. Not unheard of, but not worth overthinking since I'm replacing the risers/manifolds anyway. and I will also proactively remember to idle the engines down for a couple minutes before shutting off in the future--no coming in hot and immediately shutting down the engines, whether it's what killed my engine or not LOL :cool:
 
My 7.4's use the four slot gaskets, but your manifold / elbow pic show a 90 degree nipple on the top of the manifold and a nipple on the riser that I don't have. Where do those nipples get plumbed to? Attached are service bulletins regarding the gaskets and water intrusion.
 

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  • Service Bulletin Risers Gaskets.PDF
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  • Service Bulletin waterinengine.PDSF.PDF
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Well now there's something you don't see every day....both banks apparently simultaneously blew the head gasket between the rearmost three cylinders LOL It doesn't look like there's anything wrong with the heads/valves but I'm taking them to get magnafluxed just to be safe, then a 3 angle valve job. I'm also putting on new exhaust risers, manifolds and cooling hoses while it's torn down this far, and tearing the heat exchanger down to make sure it's clear.

Seeing as how I was running great prior to the refueling and backing upriver against a strong current, I'm still thinking at that point I sucked some water into 5 and 6 via the exhaust valve and then on the compression stroke the hydraulic pressure deformed the two equally weakest points--the spot between the adjoining cylinders. From there it was just a short matter of time before the now deformed spots blew out.

Great post and comments. I learned a bunch just reading all the sgestions and your trouble shooting.
Good luck.
 
Pretty sure you have the same setup my boat originally had. The original manifolds were blocked off from the stainless steel riser with a stainless steel plate....visible between the riser and the manifold in your picture above.

I replaced the riser and manifolds years ago as preventative maintenance.

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Thanks for the pics and info Play Date... I was under the impression that only dry exhaust used the full restrictors! Looking at the picture of my manifold/riser, it looks like the two hose connections on the manifold would be coolant in/out, and the connection on the riser is then raw water that gets dumped out with the exhaust :) Sweet! Thanks again for the heads up
 
Thanks for the pics and info Play Date... I was under the impression that only dry exhaust used the full restrictors! Looking at the picture of my manifold/riser, it looks like the two hose connections on the manifold would be coolant in/out, and the connection on the riser is then raw water that gets dumped out with the exhaust :) Sweet! Thanks again for the heads up

That is correct. When you separate your riser from the exhaust manifold you should find a thin piece of stainless steel blocking off the ports. Sometimes that fails and causes antifreeze to leak into the riser and exhaust manifold. You can reuse it if the metal is okay. In my case I opted for a replacement piece made of thicker stainless steel sandwiched between two gaskets.
 
Thanks for the tips! I have actually been shopping for and getting ready to do an order online for risers and manifolds for both engines, but didn't realize there were different options available other than just plain cast iron :) The only reason I hadn't ordered yet is because I notice when you shop for risers that there are three different gasket choices, and I'm not sure which one we use. There are full open and half open for our wet exhausts, then full restricted for dry exhaust (that we don't use). Do you remember if our manifold to riser gaskets have all four slots open, or have two open and two with the small holes? I was going to take one apart this afternoon to see, but you could save me some time LOL

On the root cause...I decided to not overthink it. I know it was water intrusion back through the exhaust valves, and the manifolds/risers are due for and getting replacement either way. I lean away from the known leaky exhaust issue just because it happened to both head banks simultaneously as I was docking for fuel, not just one side or the other. For both banks to have a simultaneous part failure on an engine that was running perfect, now THAT would be a crazy coincidence. Not unheard of, but not worth overthinking since I'm replacing the risers/manifolds anyway. and I will also proactively remember to idle the engines down for a couple minutes before shutting off in the future--no coming in hot and immediately shutting down the engines, whether it's what killed my engine or not LOL :cool:

Looking at your pictures above, tell me that your manifolds are cooled with antifreeze your elbows are cooled with raw water.
Mine is the same..
You need the full block off gasket, there are no holes in this gasket whatsoever .
 
My 7.4's use the four slot gaskets, but your manifold / elbow pic show a 90 degree nipple on the top of the manifold and a nipple on the riser that I don't have. Where do those nipples get plumbed to? Attached are service bulletins regarding the gaskets and water intrusion.

His manifolds are part of the fresh water or antifreeze cooled system.
His elbows are raw water cooled.
This is referred to as a full fresh water cool motor.
FYI, elbows are always raw water cooled.
 
Pretty sure you have the same setup my boat originally had. The original manifolds were blocked off from the stainless steel riser with a stainless steel plate....visible between the riser and the manifold in your picture above.

I replaced the riser and manifolds years ago as preventative maintenance.

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That will work. Not familiar with that brand tho...The OEM is a one piece silver gasket installed without any sealer.
 

Yup....that is the one Mercruiser recommends. My problem is that on four sizeable spots there is 15-20 lbs of antifreeze pressure. After pulling apart dozens of these things....the Mercruiser block off gaskets distort and in some cases weep antifreeze into the raw water jacket. When a customer says I'm losing antifreeze and it isn't in the motor.....it is one of the likely culprits which is why I use something else.
 
Yup....that is the one Mercruiser recommends. My problem is that on four sizeable spots there is 15-20 lbs of antifreeze pressure. After pulling apart dozens of these things....the Mercruiser block off gaskets distort and in some cases weep antifreeze into the raw water jacket. When a customer says I'm losing antifreeze and it isn't in the motor.....it is one of the likely culprits which is why I use something else.
Yeah, the stainless plate with a gasket on each side seems like the bulletproof setup that will last a long time in freshwater only, WAY better than any single-ply flexible gasket. Definitely the route I'm taking on both motors, thanks again for the visuals
 

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