What are the symptoms of cracks in an Exhaust Manifold?

[KM7]

5.8 Ltr Ford, OMC OD. 1991

I'm changing the risers/elbows. I already have those. I am considering replacing the exhaust manifolds as well. The motor has been running fine. It starts easily, has plenty of power, and doesn't burn oil. Just to be sure, if there were cracks in an Exhaust Manifold, what would the symptoms be?

Thanks

 

[alldodge]

Cracks in manifolds would either leak outside where you can see it or put water in the cylinders

If your a fresh water boater I doubt your Mans need replacing

 

[Bondo]

Ayuh,..... As I said in yer other thread with pictures, these manifolds are Junk,......

 

[KM7]

I'm in saltwater, Southern California. This boat was in a lake for the first 21 years, then in a saltwater marina for 4 years. I've had it for 9 years. I keep it on a trailer and always flush it out after use. I have also been using Salt-Away for the last 5 years. The risers were pretty rusted, and I had a partial clog in the hose feeding one exhaust manifold. I'm planning on changing them but I would still be interested in knowing what the symptoms are of a leak or crack.

Thanks

 

[alldodge]

I'm in saltwater,

Then as Bondo said they are junk

 

[Bt Doctur]

too much cast iron removed from the water passages, these cause water to enter the manifolds/cylinders

 

[KM7]

How can you be sure that I would get water to enter the manifolds/cylinders? I can have the manifolds cleaned, resurfaced and pressure tested.

 

[Lou C]

There is just not enough straight cast iron left in the sealing surfaces of the manifold for the gasket to seal. You need to look at a new one to see how it should look. The wet joint design is very prone to leaking there unless the sealing surfaces are perfect.

 

[Lou C]

Here's a new one, see the difference?
When the time comes I will replace both on mine as well. They aren't leaking now but what is the point of putting new elbows and gaskets on an old manifold that might start leaking in the next season? You might not see it leak on the outside but then, you'll get a misfire, rough running, etc because salt water rusted your valve stems. Or, it could be catastrophic, a full hydro lock. Motor will be locked up, cylinder walls rusted. Time for a new long block for 5 grand. And still, NEW manifolds/elbows, etc!
This is why experienced boaters in saltwater have outboards. Or when they repower their I/O or inboard, they spend the extra money for full closed cooling. These are the only ways to get away from this problem.
When I change mine next (probably this fall or next spring) it will be the FOURTH exhaust system I have put on this same engine, in 25 seasons in the salt. (2 OEM OMC batwings, 2 Barr Marine 2 piece systems)
BUT, no hydro locks or ruined engine due to bad exhaust.