cracked exhaust manifold out of nothing ???

[aerobat]

hey gents !

we are on holiday with our 240 sundancer 5.0 mpi , steaming inland channels towards the baltic sea . the tour takes about 5 days / 6 hours or ride daily until we reach the baltic sea .

on day 5 suddenly the stbd exhaust manifold cracked , without any previous signs .

i payed a premium to find mechanics and parts but had no other choice .

there was only an outside crack and no water in the oil , no signs of water at the exhaust ports . new manifold installed and we are back on the water.

my question is : how the hell could that happen midseason after logging about 40 hours on the boat this year previously such suddenly ?

the risers look good , anything else is fine.

thanks for any suggestions

 

[alldodge]

How many hours on the exhaust ?

 

[aerobat]

365 hours . sitting in fresh and once a year salt for 3 weeks but instantly flushed after holiday .

 

[Scott Danforth]

Theory 1. manifold had a prior crack in it

Theory 2. lack of water flow, manifold over heats, then what ever caused the lack of water flow allowed water to hit the hot manifold and instantly crack the manifold

look at your cooling system

look at all the rubber bits in the exhaust system

 

[aerobat]

i,m leaning to theory 2 . we were trolling inland channels all day long at 1500-1700 rpm . then we reached a no speed limit zone and i opened her up . shortly after this the crack must have happened .

maybe an obstruction and hot manifold with plenty of cool water after pushing the throttle down .

with theory 1 ( freezing over winter ) i do not believe i would have made half a season.

the engine temps were never at or near redline .

i never heard before you can burst a manifold trolling all day long and then gunning the engine .

after replacement the ( mercruiser certified ) mechanic ir-gunned the manifolds and risers saying its all good ... but i lost confidence going open sea

 

[alldodge]

Stuff happens and many times the real reason is never known. Trolling all day then running had should not have caused the issue, maybe just a manufactures defect that took this long to show up

 

[achris]

You're not the first one I've heard of that has had a 'spontaneous' crack in an exhaust manifold. My neighbour had his crack internally, took the engine out with it (350MPI)... I did the repair on that, full engine strip and rebuild including 5 thou bore. Wasn't cheap. In parts and machining costs it was over $5,000 (and no, just buying another long block is not an option in Australia, like it is in USA )....

Chris.........

 

[aerobat]

well , hopefully no issues in future . i would never ever expect this to happen and i do not like not knowing the reason . i was lucky it cracked only outside .

365 hours ... amazing lifespan .