Why replace exhaust manifolds if flushed after each use?

searay205

Member
Nov 27, 2016
64
Houston
Boat Info
2011 Searay sport
Engines
5.0MPI Catalyst
I see tons of people blindly recommend replacing manifolds every "x" years. If a person has a boat which is not stored in the water and flushes after each use 99% of the time the manifolds see fresh water. I live on the gulf coast and have manifolds last in excess of 12 years. Hey if you have the pre cat manifolds no big deal since they are cheap. If you have the catalyst manifolds, holy cow we are talking thousands of dollars....
 
My buddies 2005 Rinker 350Mag manifolds just cracked (hairline) in one of the corners, external leak. So his manifolds lasted 11 years. I busted them apart and what happened was sediment built up and it overheated in an area of low flow. Flushed each time and used extensively in salt water. Very little to no erosion/corrosion behind exhaust ports which shocked me at 350 hours. Total replacement including hardware, risers, manifolds $730. About 5 years ago his boat kept overheating. To make a very long story short salt had grown around the hose that supplies water to the engine. It had collapsed it, like a giant stalactite. Most incredible thing I have seen on a boat. Found it by sticking a camera up in hose. Make sure you wash exterior of engine off also.
 
It seems that careful and consistent maintenance is the key. I have a 12 year old boat and no record of any replacement of manifolds or after coolers. My temps are consistent 175F.
I regularly change out zincs and look carefully for signs of external rust at joints. Boat lived first 10 years in salt and is now in brackish water. Thinking of pulling them this year for cleaning but not planning to replace until it’s condition indicated.
I might feel differently if I spent time off shore and operated the boat in extreme conditions to avoid a dangerous breakdown. Just my 2 cents
Happy New Boating Year Everyone!
 
I don't know what is different about them, but the diesel engine exhaust system seem to hold up much better in salt water.

On the flip side for us freshwater boaters - between myself, my father and his father we have had a dozen different boats with different engine manufactures. I have never replaced a riser or manifold nor have I ever heard of anyone around the lakes where I grew up replacing one due to rust or corrosion. Other things happen, freezing, overheating etc, but never replaced them as a maintenance item.
 
With only 16 posts, you haven't been here long enough to see one of the posts containing the following:

Flushing does wash remaining salt water out of the exhaust system but it does not stop the interior of the iron castings from corroding. The reason is because the air around a coastal boat is salt laden. Even if the boat is stored on a lift or trailer, if it is close to the coast, the ambient air is humid and full of salt. The exhaust system is open to the atmosphere when the boat is out of the water, so rust can and does occur in the cast iron exhaust manifold , spacer and elbow or riser.

There is no iron clad rule because everyone's circumstances are different, but after you change a set of manifolds on a seawater cooled engine, if you use new OEM Mercruiser castings, you can usually get 5 years out of the replacements; HOWEVER, you should remove the risers and inspect the interior cavities on year 3 and every year thereafter until replacement is needed. If t hey are free of rust, salt deposits and sediment, you have invested in a set of gaskets, and you can continue to use them, as long as you open the system and inspect the castings annually.

Risers and manifolds usually rust from the exhaust side in, not the seawater side in because the ambient air containing salt in the moisture continues to rust because the exhaust system is open to ambient air from its exit point all the way to the head, even if the water side is flushed after use. When the rust formation get so heavy that it forms flakes, in the exhaust passages, you are on borrowed time. At some point, one of those flakes breaks away and exposes a pin hole in the cast iron underneath. When that happens, you can easily fill a cylinder (or engine!) full of seawater, and that usually happens as the engine cools so you dont know you have a problem until your next time out. Bottom line here, is that a set of manifold/spacer/riser gaskets is a lot cheaper then a valve job or an engine overhaul, assuming you catch the leak before the engine freezes up from rust in the cylinders, so check the castings every year, after year 3.

As far as this quote from your second post:

"........Flushed each time and used extensively in salt water. Very little to no erosion/corrosion behind exhaust ports which shocked me at 350 hours. Total replacement including hardware, risers, manifolds $730. About 5 years ago his boat kept overheating. To make a very long story short salt had grown around the hose that supplies water to the engine......"

Respecfully, salt deposits would not have formed inside the engine, if the engine had been properly flushed as you indicated.
 
Salt air is crazy. I can park my car at my dock for a weekend while we are out on the boat and come back to find a film of salt on it. I almost never take my 68 GTO or C6 Corvette down to the dock for just that reason.
I hose down my boat at the dock every few days when I don’t take it out, and thoroughly wash and rinse it every time I return from taking it out.
I had Mercruiser 5.7’s in my old boat and replaced the exhaust every 5 years with OEM parts.
Frank is absolutely right. Pull those risers after 2-3 years and give them a good look. Gaskets are cheap and it’s a pretty easy pre-season job. I could do both sides on both motors on my old boat in an hour or so start to finish.
Better safe than sorry. I’m also a believer in OEM parts.
My kids have an 18’ Maxum with a 3.0 Mercruiser we picked up at the beginning of the 2016 season. They used it that summer, we winterized it, put it away, and I replaced the aftermarket exhaust with OEM before launching it before the 2017 season. The PO owner told me it was only a few years old but I did it as a maintenance item anyway just to be safe. The exhaust I took off was shot. I should have done it as soon as we got the boat. I took a stupid chance not pulling it because it was supposedly pretty new. We’re lucky we didn’t have a problem.
A close old friend retired from his auto repair business about a year ago. He’s probably bought and sold a few dozen twenty something foot I/O boats over the years. He’d buy them with bad motors, fix them, and use them for a season or so until someone offered him the right price. He almost always had a boat and made money on every one he ever owned. He mostly got them from customers who had them sitting in their driveways and just wanted out of them.
Every boat he ever bought with a bad motor was a result of an owner letting the exhaust go too long. He is kind of a frugal guy, but he always believed in putting OEM manifolds and risers on his boats because he saw too many aftermarket ones become junk after 2 or 3 years.
 
My previous boat had twin 5.7s in it and tired looking Barr Marine manifolds and risers on both engines.

I found documents on board that indicated that they were 8 years old at the time and the boat had until a year before been in the Chesapeake.

Since these were OMC engines (ugh), OEM parts were more expensive than Merc stuff and harder to find, so I went with Barr again and was impressed with the machining at the gasket surfaces and the quality in general.

I ran it 5 years in fresh water and the new owner ran the crap out of it last season with no problems at all.

Conversely, I’ve seen GLM manifolds and was horrified. Looked like kids made them in shop class.
 
I see alot of imfo on saltwater ran boats but what about fresh water boats that are trailered? I have a 77 searay sda 240 weekender with twin 470's. I just bought this boat a few months ago and haven't had it out yet been fixing it up and doing a few upgrades. I do run it on muffs at least once a week and the temps are running at center on the gage and not overheating. I am not aware of any replacements being done on the exhaust system on these engine but don't know their history. Both engines both run very well and after performing a little work fire right up. How often should the manifolds be looked at if the boat has only been ran in fresh water? I see no indication of salt water use.
 
I see alot of imfo on saltwater ran boats but what about fresh water boats that are trailered? I have a 77 searay sda 240 weekender with twin 470's. I just bought this boat a few months ago and haven't had it out yet been fixing it up and doing a few upgrades. I do run it on muffs at least once a week and the temps are running at center on the gage and not overheating. I am not aware of any replacements being done on the exhaust system on these engine but don't know their history. Both engines both run very well and after performing a little work fire right up. How often should the manifolds be looked at if the boat has only been ran in fresh water? I see no indication of salt water use.

See my post #4. Freshwater manifolds generally last forever, but on a boat that old I might pull the risers and have a look. My boat is a 1999 always in freshwater, not even thinking about risers or manifolds.
 
While the exhaust in fresh water boats do indeed last far longer I would still invest a little time and some gaskets on a boat that was new to me just to inspect everything and establish a baseline.
 

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