Exhaust manifold questions

[Osmosis]

I am currently in the process of purchasing an 86 sundancer 260, chevy 350 alpha 1 outdrive. Outdrive was replaced last summer. I had a surveyor check the boat out and I'm waiting for the oil test results before I purchase the boat.

I know a major cause of engine failure is due to the exhaust manifolds going bad. I was told the sea ray I'm considering has a "closed cooling system" and that the sea water will not go through the exhaust manifolds. I don't quite understand how the closed system works. I know there is a riser and the riser on my boat needs to be inspected as it has some minor surface rust. I know from experience, a little rust can be a huge sign. So, how what is one to look for for indication of a failing closed exhaust system? This boat was on a lake it's entire life till 5 years ago. During the past 5 years, it was in salt water 3-4 months out of the year and then in dry storage.

Thank you

 

[J Levine]

On gas inboard motors there are two type of closed cooling systems, often called a "full system" or a "half system" In both cases the heart of the system is the heat exchanger which is a cylindrical bundle of brass tubes. Sea water is circulated through the brass tubes from the engines raw water pump while coolant flows around the tubes from the engine block, pushed by the engine mounted coolant circulation pump. In both types of systems once the raw water exits the heat exchanger it is routed to the exhaust system for cooling purposes and then goes overboard with the exhaust. On a "half system" the raw water passes through both the exhaust manifolds and then through the exhaust elbows (often called "risers") before being mixed with the exhaust. On a "full system" the exhaust manifolds are plumbed into the closed circuit of the engine block and are cooled by the same coolant. At the same time on a full system a block off plate is used between the exhaust manifolds and the elbows and the elbows still cooled by sea water.

The maintenance items on a full system include: the elbows that will eventually fail from corosion and need to be replaced every 5 years or so in a salt water environment, the gaskets used in the exhaust system also need to be replaced every few years. The heat exchanger may have a zinc that needs to be changed every few months. The heat exchanger also needs to be cleaned from time to time and eventually will have to be removed and taken to a radiator shop to be boiled out and cleaned. The only real way to check the elbows is to removed and inspect them.

 

[Osmosis]

Thank you for your reply, it kind of makes sence to me. I'm a car guy, just starting to learn about boats. I'm sure once I figure out the differences, I'll be "on board"

I remember the surveyor telling me about the "sacrificial Zink" in the heat exchanger but my heat exchanger has antifreze in it not sea water, unless there is sea water in the copper tubes surrounded by antifreze? Regarding my risers, I was told that there is a sandwhich type gasket between the riser and the exhaust manifold. Is that a block off type plate between the riser and exhaust manifold? I guess now, it's a matter of taking it apart. I figure that I will take the entire exhaust system off and replace any parts that need to be replaced. I do see some surface rust on the ehxaust manifold's, but it does not look lke anything major. I was planing on taking the manifolds to a machine shop who i'm in good with and then having them magnaflux them and remove the rust. That will not cost me a cent as I've spend thousands on my hot rods I will have to pay for power coating (at a discounted price) if I choose to do it. It's Nice to have friends!

My main concern here is that I have read one major cause of engine failure for a boat is due to the exhaust manifolds leaking. Is this much of a concern with a fully closed system?

Thanks again!

 

[J Levine]

In my opinion I think gasket failure probably contributes to engine damage as much, if not more than, rot through in a casting. This is especially true with the Mercruiser gasket design which sucks. My boat has full systems. I installed all new manifolds, elbows and 3" risers in 2004. I now have 6 seasons of use on these parts in a salt water environment and I have replaced the gaskets 3 times, the last time being last weekend.

 

[Osmosis]

Thank you for the advise! I'm gonna take the manifolds off when I get her home...once I get the results from the oil test. I am getting this boat from the orginal owner who has every maintence record on it and it was very well taken care of.

Consequenlty, we live across the street from a lake that has a very high iron content. I was amazed to see how fast a pair of steel siccors rusted in a two month time period. That being said, I know if my boat has any rust exposed on the exhaust ( or anywhere else), the rust will spread even faster so I gotta get rid of the rust asap! I may even pull the heads and freshen them up too.

 

[Osmosis]

Can you point me in the direction to find the best gaskets at teh best prices for this application? sounds like you 've done this a few times:thumbsup:

 

[J Levine]

Get the Merc OEM gaskets, they have been through many revisions and have the best gasket. What sucks is the design of the manifold and elbow water jackets and the very thin separation between the exhaust and the water. Merc has addressed this on their current small block GM products with a design they call "dry joint" If you have to replace the manifolds and the elbows you can upgrade to this newer style.

 

[purpony]

My father owned a 86 340 and one of the riser manifolds sprung a leak in the closed system. Ended up with water in the oil an hr from home. Ended up coming home on one engine. Changed the oil about 8 times in that engine to remove the water and replaced the riser. Never another issue and the boat is still running strong on its original 454s from 86.

 

[Alegria]

Thank you for your reply, it kind of makes sence to me. I'm a car guy, just starting to learn about boats. I'm sure once I figure out the differences, I'll be "on board"

I read that the antifreeze system is like your car and cools the block, while the water system acts like the air from the fan in your car and carries the heat away. That analogy made it all make sense to me too!!!

The raw water depending on the type of install may cool other things too, in my case transmission and oil...

Good Luck!!

 

[stevea661]

The guy from San juan engineering told me the coolant runs through the tubes and the raw water around them. When I installed mine I had to do some modifications and in talking to him, because I like to know how stuff is put together, I asked him what flows where and how.