Manifold advice

[Will 55]

My boat is freshwater only, relatively low hours for an '87, and the manifold is a bit rusted and has been leaking slightly for a few years. Engine has been well maintained and running great. It seems to be cooling ok, though one side seems a bit hotter than the other. I've been afraid to try and change out the manifolds and risers because the job would be tough due to the tight engine compartment, but mostly due to the fact that more than likely, at least one or more bolts will break since these are the original manifolds on a 1987 Merc. Any advice? 5.7 L merc, alpha 1

 

[sbw1]

You could pay a competent tech to perform the work. It is somewhat unusual for a fresh water boat to required new manifolds but not unheard of. Would likely be the last time engines this age would require new ones again.

 

[northern]

Take the risers off and look. If good take them to a rad shop and get them cleaned and put them back on with new gaskets. If bad you just saved your engine. I got new manifolds and risers on a twin V6s for $6,000 CDN. There was little room as boat beam was 10.5 feet.

 

[Will 55]

Since it's cooling ok (gauge never more than halfway) and it's only leaking slightly on one side (just a few drips) I've been taking the attitude of 'if it's not broke why fix it?' Since I only run my boat maybe 20 hrs a year max, maybe it'll be ok? It should be easy to remove the risers and take a look.

 

[sbw1]

Since it's cooling ok (gauge never more than halfway) and it's only leaking slightly on one side (just a few drips) I've been taking the attitude of 'if it's not broke why fix it?' Since I only run my boat maybe 20 hrs a year max, maybe it'll be ok? It should be easy to remove the risers and take a look.

Never hurts to look. You stated you didn't want to mess with it. If you know how, do it.

 

[techmitch]

Since it's cooling ok (gauge never more than halfway) and it's only leaking slightly on one side (just a few drips) I've been taking the attitude of 'if it's not broke why fix it?' Since I only run my boat maybe 20 hrs a year max, maybe it'll be ok? It should be easy to remove the risers and take a look.

"the manifold is a bit rusted and has been leaking slightly for a few years. Engine has been well maintained"
NO! It hasn't been well maintained if it's been leaking for even a week and you have ignored it.


"Maybe it will be ok"
NO! It won't be ok. Fix it or your going to cost your self an engine.

 

[Lazy Daze]

If you wait till it "breaks", you can also break your engine.

But let's back up a bit... maybe you don't fully understand how a manifold/riser is designed or it's purpose. There are passageways built-in for BOTH water and exhaust gasses. Is some places, the wall between the two is not very thick. In your case, you may just have a failing gasket. But it could be failing on the inside, as well as possibly the metal degrading. Open it up and take a look.

Fresh water it's whole life?

 

[Will 55]

If you wait till it "breaks", you can also break your engine.

But let's back up a bit... maybe you don't fully understand how a manifold/riser is designed or it's purpose. There are passageways built-in for BOTH water and exhaust gasses. Is some places, the wall between the two is not very thick. In your case, you may just have a failing gasket. But it could be failing on the inside, as well as possibly the metal degrading. Open it up and take a look.

Fresh water it's whole life?

Yes, freshwater it's whole life, never once in salt. The slight amount of drippage on one side is near the bolts. If the manifolds were failing internally, wouldn't I be able to see it as water in my oil? That has never happened. There is some visible rust on the outside of the manifold, but not a lot. Just 4 bolts for the risers and how many for the manifold?

 

[KevinC]

If it's a "wet joint" type manifold and it's leaking then the gasket could be compromised and if the gasket is compromised you face the potential for water intrusion into the engine. It does not take a whole lot. Once water get down into the head and fouls a valve or seat you may start to lose compression. In that case it's not necessarily a blown engine but you will at least a full head job.

-Kevin

 

[BillK2632]

Generally the manifolds and risers on a freshwater boat last the life of the engine. I have owned a half dozen boats all freshwater, and have never replaced a manifold or riser. You just never hear of that around the lakes I boat on - salt water, even limited use is a whole different story. That said, if I ever saw rust on the outside of the manifold or evidence of water leaking anywhere around the exhaust system I would get to the bottom of it ASAP. A failing manifold or riser is a silent engine killer - it usually does it's dirty work after you shut the engine down. Water drips into a cylinder through an open exhaust valve. A little leak you get rust and corrosion and you are in for new or re-worked heads and of course new risers/manifolds. A big leak and you try starting the engine with a cylinder full of water and something big breaks - now you are looking at a total rebuild or replacement. By the time your oil shows evidence of water, the damage is probably done.

So, bottom line, do freshwater boats generally need manifolds / risers inspected and replaced like salt water - no. Can things happen - yes.

Get to the bottom of this or your probably looking at a big problem.

 

[Lazy Daze]

Yes, freshwater it's whole life, never once in salt. The slight amount of drippage on one side is near the bolts. If the manifolds were failing internally, wouldn't I be able to see it as water in my oil? That has never happened. There is some visible rust on the outside of the manifold, but not a lot. Just 4 bolts for the risers and how many for the manifold?

If you wait till you see it in the oil, it could be too late. Obviously this is your call - but best practice is preventative maintenance. As mentioned, it could just be a gasket - but you don't know till you pop it off.

You don't need to take the manifold off.

 

[northern]

If the risers are original there may be a brass 1/4" plug in the top. If there is take it out and take a wood BBQ secure and put it in until it hits the bottom. There is a depression in our manifold that holds about 3/16" of water. I found antifreeze once in mine. The gasket between the riser and manifold were leaking.