7.4 Merc Exhaust

Russ Calasant

Russ Calasant

Active Member
Dec 9, 2009
2,128
East Greenwich, RI
Boat Info
2001 480 DB
Engines
CAT 3196
Has anyone experienced the water ingestion problem or not thats inherent to 7.4 Mercs and found a solution? It has cost me at least one engine maybe two. Although I have come to terms with the replacement after I have completed regular recommended maintenance and replaced every three yrs with OEM whether needed or not the exhaust still failed. This failure is due to an obvious design flaw that is not acknowledged by Merc but is by tens of thousands of boaters. My concern is having to do it again, how crazy is it that people have have done it 2, 3 or more times?! Merc has developed a dry joint exhaust as a solution for newer motors but no application for 7.4s.
Please tell me that there is a better solution than cracking and inspecting the exhaust every year. I was always told that you can't see all even when you do and now I have proof!
 
The dry exhaust still won't solve the problem with this issue. It was apparently only a problem with the High Output 7.4's and it involved the exhaust setup itself in combination with the engine.

The exhaust did not use a water lift sytem, but instead an exhaust "log". This allowed the water line to be quite close to the level of the exhaust manifolds and provided more of a "straight line" access for the water to be forced back in towards the engine under the right conditions.

To make things worse, the high output engine has a more aggressive cam which allows more valve overlap for better "breathing" especially under high revs. It does however allow some time with both the exhaust and intake valves open at the same time during the intake stroke. That's fine to suck some spent exhaust along with the fuel/air mixture, but with the water being so close to the manifold it could suck more than exhaust fumes. It could suck some water - bad for the engine!

The solution was to install water lift mufflers, which prevented the water from backing up as easily AND installing taller exhaust risers to get higher from the water line.

This is what I recall reading about the issue and I'm no expert, so you might benefit from doing a search within CSR! Good luck.
 
Thanks Jim, I have done a lot of reading and am more confused! I am not positive of the exact cause, just what I am told in my case. I have water lift exhausts and the risers are as tall as they can be. An exhaust valve seat coroded and the remnants damaged the piston and cylinder wall. When the other half was checked there was evidence of water ingestion but not catastrophic. The only evidence ever found was during an inspection last year it was minor but an obvious indicator that one exhaust port had seen water. I only replaced the manifolds but the damage had already been done so the inspections did not indicate until it was too late to prevent the damage. However i do understand at that point i should have pulled the heads for further inspection that would have prevented catastrophic failure. For 600 hours there was no evidence at all with annual inspections and regular replacement every 3 yrs, OEM parts only. So the question remains, what do I do differently going forward? I can't replace the motors and go forward with same practices and expect different results, isn't that the definition of insanity!?
 
Russ
Are you certain the problem was caused by SEA WATER and not coolant leaking past the elbow/riser gaskets? If you have water lift mufflers, I can't understand how seawater would get back in there. Did you have any instance of cranking and cranking that engine for an extended period of time trying to start it? That could cause seawater to pump through the system and fill the mufflers, which could lead to a back up into the system, unless you had a valve train problem that allowed water in.

Which cylinders were rusted? If it was 2 and 4 or 3 and 5, the problem could be from a bad mating surface on the old exhaust system (at the manifold/riser interface), which is what got me. Coolant from a leak like this will flow down and start to fill the aforementioned cylinders.

I corrected this problem on my stbd engine two years ago with all new merc exhaust components, but the remnant of it is lower compression on 2,4,3 & 5. Heads are coming off this winter, all in the interest of preventing disaster.

My problem was originally discovered with an oil analysis and confirmed with removal and inspection 2 years ago. And like you, I should have done the heads THEN. Luckily for me, it didn't end up with engine death and destruction, but now I get to work twice >:-(
 
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Russ, from your reply it sounds like your problem is unlikely to be caused by the original design flaw. You already have the water lift mufflers and taller risers.

Are your engines closed cooled (with antifreeze) or raw water cooled?

Sorry to hear of your troubles and yes, I'd want to have the mystery solved too, especially before investing in new engines! You might want to see if there are any "dry joint" exhaust manifolds available for your engine if water intrusion is suspected. I think it's crazy to be replacing all your manifolds/risers every 3yrs! You might even look into an Australian company that makes one piece stainless manifold/risers which should solve any "leaky joint" issues.

The company's name is Hi Tek and there is at least one CSR member that has these on his boat. http://www.hitekmarine.com.au/hi-tek.htm
 
Russ, there appear to be two different issues. One is water injestion and one is water reversion. As I understand it the injestion is actual water getting back up the exhaust. Sudden de-acceleration, backing down, following seas, flappers not installed when they should be.

Reversion is a function of the camshaft cut (valve overlap) which allows a mist of raw water to collect in the exhaust manifold.
I had reversion and lost two engines plus 2 additional valve jobs. Severe exhaust valve and/or piston ring damage was evident.

After much reading I have done the all of the following:

Squeezed in another 3 inches of spacer
Except when docking I idle above 1500 rpm. (one engine off to keep below the speed zone limit)
Idle at 1500 rpm as I turn off the key
Added the exhaust resonators in the down tube
Installed brass plumbing valves in the access plug on top of the risers. (after every shutdown I fog thru the valve with wd40)
Plumbed in a fresh water flush system to flush the spacers and riser. ( I have closed cooling).

Don't know if any or all is helping but so far I have not had a problem.

There is also an issue with the casting design of the risers. Apparently only the original OEM design was correct (no longer available).
I keep watching for someone to start casting with the old design.
 
great, I purchased a 1998 330 sundancer a few weeks ago. 420 hrs twin 7.4 mpi's
next time im at marina i will measure things and take a closer look at the setup
 
Russ,

I tried to give you a call a couple times - you're voice mail is full, so I couldn't leave a message.

One thing we need to look at is the actual descent angle on the exhaust hoses. Mercruiser strictly defines the minimum allowable angle, and the length for which it must be maintained, in their specifications. Improperly configured water lift mufflers can contribute to your problems. I consider Mercruiser specifications a minimum, and work to exceed their requirements by a substantial margin during refit.

Water lift mufflers can be modified for improved performance in several respects, but only when using the proper resins and design. You already stated that you have the maximum amount of riser height for your engine room, so that's a good start. Beyond that, we need to take several measurements of your exact configuration before any conclusions can be drawn.

Dale
 
Thanks guys, all very helpful, I obviously don't know enough about this. I need to determine exactly what the cause was before I can find a solution.
Happy hour, it seems that you have taken many precautions to address both the issues I have been reading about that have been scaring the **** out of me. I like your ideas. I believe my problem is/was "reversion". I think my problem occurred over many years and hours, resulted in gradual corrosion, provided warning and ultimately failure.
Dale, being the expert that you are, I am sure you will be able to ask the right questions and after inspection come to a conclusion pretty quickly. Then be able to provide solutions that will instill confidence going forward. I will clean out my VM, sorry I missed your call! Let me know next time you will be in Mystic.
 
I have a 2001 230 with 454 MPI Horizons.
Just heard about this water injestion issue and wanted to see what can be done to correct it.
One guy I talked to about it said that he had a 1998 that Mecruiser paid for it in 2009 to be done.
I called Searay to see about this, he is getting back to me.
He asked for the HID of the other guys boat which I do not have access to.
Waiting for hhim to research and get back to me, seems like if they fixed it from 1998 to 2009 that I too should be covered...hopefully.
I also asked the question, do I wait for the failure or do they replace prior to failure?
(the other guy said they replaced both engines and exhaust system for free) will see what happens,,,
Jay
 
j0256 said:
One guy I talked to about it said that he had a 1998 that Mecruiser paid for it in 2009 to be done.

seems like if they fixed it from 1998 to 2009 that I too should be covered...hopefully.

(the other guy said they replaced both engines and exhaust system for free) will see what happens,,,
Click to expand...

Jay - First off...Welcome to CSR! By far the best site on the internet for all things Sea Ray & boating in general. The experienced guys (myself not included) that post here are fantastic. Ask the questions & they provide good, sound advice.

As for "this other guy" - I seriously doubt Mercrusier paid this guy anything. That is just my opinion, but I've got $100 that says I'm right.
 
I had this exhaust issue with my 1998 370DA and in 2005 and me being the third owner I questioned Sea Ray about this design flaw and they gave me the part number for the fix that I could buy from my local dealer. Turned out the part was the entire $2800 water lift muffler system. I called back Merit Island and complained a little and a few days later a giant box was at my doorstep with the complete system compliments of Sea Ray. I was thoroughly impressed by this gesture which is one of the reasons I'm on my fifth Sea Ray. I highly doubt they would still do this but they really stepped up for me when I needed them. Mark
 
Very good piece and sheds a lot of light on the problem. These issues were exacerbated by the move to underwater exhaust (early to mid 1990s?), mainly with the latest generation of the 7.4, where GM move towards the "aggressive" valve overlap as described in the article - wonderful for an automotive application but sometimes disastrous in marine.
 
Great article thanks for posting it. My last two boats had the 380 hp Horizons and I had a reversion issue with my DA but no problems at all with my AC. Now I have a 1999 330DA with the 310hp MPI engines and thought I wouldn't have to worry about this anymore but this article has me thinking about it again. My boat has 360 hrs since new but it has already had both engines replaced one in 08 and one in 10. The motor that was replaced in 08 was bad when I bought the boat so I have a new long block in my garage right now getting ready to be put back in. The manifolds on the bad motor were pretty ugly so I hope these motors were destroyed due to lack of maintenance and not something else. The only elbow listed in the article that was considered to have an effective vacuum break was one with a 14 degree down angle. Not sure if I need 14 or 7 degree for my boat. I'm starting to regret my decision to buy another big block inboard Sea Ray. Mark
 
Mark
I don't think you need to worry. The 330DA that vintage is pretty bulletproof, despite having log mufflers. Most of these 454/7.4s fail b/c people don't keep an eye on the gasket btwn the elbow and the manifold and water trickles down in there and eventually, if more accumulates in the exhaust gas passages than can be blown out, you get hydrolock. The dry joint design and the stainless elbow of the 8.1 has solved the problem that has been damaging big blocks for decades. I think your engines were more likely the victim of neglect.
 
I
rondds said:
Mark
I don't think you need to worry. The 330DA that vintage is pretty bulletproof, despite having log mufflers. Most of these 454/7.4s fail b/c people don't keep an eye on the gasket btwn the elbow and the manifold and water trickles down in there and eventually, if more accumulates in the exhaust gas passages than can be blown out, you get hydrolock. The dry joint design and the stainless elbow of the 8.1 has solved the problem that has been damaging big blocks for decades. I think your engines were more likely the victim of neglect.
Click to expand...

Ron, I hope you're right about that. I'll surely be keeping a close eye on these two motors for a while.
 
Sublimetime said:
searay Mark-----Do you have the 1/2 fresh water cooled or the full system?
Click to expand...

I currently have full raw water cooled. This is my 5th Sea Ray with big block inboards and the first one that has been raw water.
 
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