Exhaust manifold replacement

B

Bradb571

Member
Oct 8, 2007
93
Florida
Boat Info
340 SeaRay Sundancer 2005
Engines
Twin Inboard 8.1L
How long should I expect to get out of my manifolds? I'm in saltwater in SW Florida. I have a 2005 340 Sundancer with 8.1's
Thanks for the input
 
3-5 years.
 
Agree with Mike. Maybe more but just not worth the risk.
 
On my last gas boat I mudufued the cooling system. Made it a closed system. Kind of a pain but never had to mess with the riser issues.
 
I'll check but I'm thimking not. It was a twin mercruser chaparral. The swap over was straight forward. Water was still drawn from the out drive impeller through a heat exchanger that came with the kit. It was a good system.
I'll see what I can dig up.
 
Last edited:
Just been speaking to my Merc tech about this as mine are now 5 years old. He said 5 to 7 years, but any time after 5 years the risk of failure increases. FWIW, the manifolds were previously 9 yeas old before I had them changed after being flagged on the PPI for replacement. I'll change mine next year.
 
A lot depends on the saltiness of the water that your boat spends time in. Here on the west coast of Florida we live on the Gulf of Corrosion (Mexico).
 
I was sitting on the fence last season. So pulled one and inspected. Glad I did, Merc tech I showed it to said I was just 1 good run away from disaster. Bought the boat in 2015 and broker I bought it from had no idea when they were last changed. Looked good on the outside, but inside walls were very thin and one (port engine, port side) actually had eroded through. It is a crap shoot, but if you are on your 5th season, certainly deserves at least an internal inspection. In Florida, I would not go longer than 5 years. Nothing like piece of mind knowing exactly where they stand.
 
Buy a kit and swap it all over to a FULL fresh water cooling system. You will hate the expense, but love the worry free years of enjoyment.
 
Coming back with an update. I had the Merc tech pull the elbows off the manifolds and there was plenty of rust in the manifolds. So new manifolds are going on and I'm also changing out the elbows for good measure along with new manifold bolts and water drain.

I'm miffed how the exhaust manifolds have rusted in the collector, although the Merc tech said this was typical due to the exhaust setup on SeaRays. He says they draw water mist back up the exhaust tube post the elbow at idle and down into the manifold.

This prompted me to look into this further and I found a Mercruiser service bulletin recommending taking the motor to 1300 rpm for just under a minute, slowly bringing the throttle back to idle before immediately shutting down. I also found a similar recommendation was made by Volvo with their version of the 8.1 except it was 2,000 rpms and switching off the motor at those revs.

Is anyone else following Mercruisers recommendation at shut down? I'm also wondering whether I should also run a riser under the elbows - thoughts?
 
I had the motors that were available just before your model, 7.4 horizon mpi ho 385 hp. They had way to much valve overlap, for a marine motor. They were notorious for sucking water back in at idle.

I did two rebuilds before the service advisory about the higher rpm idleing. That was a great help with stopping the problem, but I think the turbulator plates in the elbow gasket, and the resonators in the exhaust tube really helped too.
 
Dear All, I bought my 250 Slx ( 2008 model ) with 2015 350 mag mpi engine ( 200 hours ) in 2018 singapore. The boat is based in Phuket - Thailand now. Last year in November i had to replace both manifolds, risers and elbows.. ( not a cheap maintenance story - but i had no other solution or it was a blown engine.. ) Now i am still considering to mount a fresh water cooling system. Unfortunately overhere we do not have the same amount of experienced people around here, so i need full information from someone who did on a 350 Mag Mpi and what kind of system was used and the price for that conversion. Then i can check on the website of the producer of the system and show this to the locol mechanics here if they can do it .

Anyone has experiece with this closed cooling system :
https://www.perfprotech.com/mercrui...um-full-system-5320-8b-5320-8b/product/243425
 
Last edited:
You must log in or register to reply here.

Forum statistics

Threads
113,148
Messages
1,427,289
Members
61,058
Latest member
Axlthepaxl
Back
Top