1975 351cu 233hp Mercruiser, riser block off plates or not?

[soulsalvation]

After a heat riser bellows replacement the mechanic thought the block off plate was causing the overheating in the heat riser causing the left (pilot side) exhaust system to overheat subsequently burning the rubber elbow connecting to the y pipe to burn.
I found that part also bizarre but after some research saw that this “block off plate “ made of metal is actually in the design schematic and sandwiched between 2 fabric gaskets that have 3 water passage holes in them where the plate does not, Whaaaaat? I’m confused.
What function does this plate have and is it possibly affecting my manifolds? The mechanic removed it and said my manifolds are now running normal temperature and said the other manifold will probably overheat because of the same issue and I should probably remove that plate as well.
My pilot side manifold always ran hotter than the other as far as I have owned this boat going on 27 years.
Is there a Grand Guru out there how can definitely give some correct and undisputed knowledge on this issue.
Thanks to all who respond.

 

[alldodge]

One side will run a bit hotter then the other which is correct.
So far as the block off gasket, unless the motor is closed cool it should not use it

 

[achris]

The block off plate is so the water doesn't just go straight up and out through the elbow, but runs through the exhaust manifold properly, so you don't get hot spots in the cast iron manifold that can then get hit with 'cold' water and crack.

This is your water flow diagram.


If you follow the diagram, you'll see that without the plate (the small hole in the plate is just an air bleed) water would enter the manifold at the back and just go straight out the elbow, instead of running up to the front and back into the thermostat housing/distribution block (where it is THEN directed to the elbow).

Chris......

 

[achris]

...The mechanic removed it ...

FIND A NEW MECHANIC!

 

[soulsalvation]

The block off plate is so the water doesn't just go straight up and out through the elbow, but runs through the exhaust manifold properly, so you don't get hot spots in the cast iron manifold that can then get hit with 'cold' water and crack.

This is your water flow diagram.
View attachment 345379

If you follow the diagram, you'll see that without the plate (the small hole in the plate is just an air bleed) water would enter the manifold at the back and just go straight out the elbow, instead of running up to the front and back into the thermostat housing/distribution block (where it is THEN directed to the elbow).

Chris......

Thanks Chris. This diagram is a bit confusing since my engine is a freshwater open system with no transmission or oil cooling. It’s a 1975 351ci 233hp Ford 2 barrel.

 

[soulsalvation]

One side will run a bit hotter then the other which is correct.
So far as the block off gasket, unless the motor is closed cool it should not use it

“Closed cool” you mean dual raw water/ coolant systems. Mine is simply a fresh water system

One side will run a bit hotter then the other which is correct.
So far as the block off gasket, unless the motor is closed cool it should not use it

The inserted diagram is exactly my engine. No oil cooling,no transmission,etc. Straight from the lake cooling system. As you can well see those block off plates are in the design but the mechanic states that these are my problem and once removed the exhaust will run cooler. So why did mercruiser insert these in the first place? I think I should test run without them and take heat readings and see what comes of it. I don’t think any harm will come of it. Thanks Alldodge for responding

 

[soulsalvation]

“Closed cool” you mean dual raw water/ coolant systems. Mine is simply a fresh water system

The inserted diagram is exactly my engine. No oil cooling,no transmission,etc. Straight from the lake cooling system. As you can well see those block off plates are in the design but the mechanic states that these are my problem and once removed the exhaust will run cooler. So why did mercruiser insert these in the first place? I think I should test run without them and take heat readings and see what comes of it. I don’t think any harm will come of it. Thanks Alldodge for responding View attachment 345409

Hello again Alldodge. Here’s another diagram of my boat’s manifold system

 

[soulsalvation]

Hello again Alldodge. Here’s another diagram of my boat’s manifold system View attachment 345410

Why design passage ways for water at the end of the manifolds to the risers then block them off? Unless this system is also used for the mixed rawwater /coolant systems. Who knows?

 

[achris]

Why design passage ways for water at the end of the manifolds to the risers then block them off? Unless this system is also used for the mixed rawwater /coolant systems. Who knows?

The original design had the water entering at the front of the manifold and leaving through the ports at the back (and up into the elbow). The reason it was changed was because when the boat was in the bow up attitude the manifold wouldn't fill properly, hot spots would develop and when the bow came down the cool water would hit the hot areas in the cast iron and crack it. The change of flow (from back to front) solved that issue, and part of that change was to put in those blanking plates. If the mechanic doesn't understand a simple flow diagram and what it means, you don't want him working on your boat...

Chris...

 

[soulsalvation]

The original design had the water entering at the front of the manifold and leaving through the ports at the back (and up into the elbow). The reason it was changed was because when the boat was in the bow up attitude the manifold wouldn't fill properly, hot spots would develop and when the bow came down the cool water would hit the hot areas in the cast iron and crack it. The change of flow (from back to front) solved that issue, and part of that change was to put in those blanking plates. If the mechanic doesn't understand a simple flow diagram and what it means, you don't want him working on your boat...

Chris...

such an old engine with some mysteries, i don’t expect mechanics to know every aspect of every engine that’s out there, this is something i’ll investigate carefully to see what happens. i did discover some tubing that was reversed to the manifold which i corrected but wether that was the cause of a baked riser bellow remains to be determined. this is the second time this bellow has been changed in the last ten or so years and the other one never. The mystery continues. thanks for responding Chris.

 

[kenny nunez]

Burning out a exhaust bellow on the same side without having a overheat condition points to a restriction in the elbow/riser itself. It could be either some rust slag or even a faulty casting causing a hot spot.

 

[soulsalvation]

Burning out a exhaust bellow on the same side without having a overheat condition points to a restriction in the elbow/riser itself. It could be either some rust slag or even a faulty casting causing a hot spot.

i’m going to put magnetic thermostats on both risers and monitor the heat as we drive around the lake slowly as to not build up too much heat and see what happens. thanks for the insight

 

[johnkom]

I have the same engine in a 1978 Sea Ray SRV200. She's a tank of a boat but the engine is rock solid and handles it well. I love it. Like having an old heavy Cadillac for highway travel. I have a Marine Holley 4 barrel on mine. I recently replaced my manifolds and risers due to age. The blocking plates are in place. Manifolds and risers run 140 to 155 degrees. Notice the schematic shows the hot engine discharge water going directly into the manifolds. So they should run pretty warm.

 

[soulsalvation]

I have the same engine in a 1978 Sea Ray SRV200. She's a tank of a boat but the engine is rock solid and handles it well. I love it. Like having an old heavy Cadillac for highway travel. I have a Marine Holley 4 barrel on mine. I recently replaced my manifolds and risers due to age. The blocking plates are in place. Manifolds and risers run 140 to 155 degrees. Notice the schematic shows the hot engine discharge water going directly into the manifolds. So they should run pretty warm.

that’s my engine! exactly! i will get to the bottom of this eventually and determine wether or not these block off plates actually serve a purpose. thanks for responding and sharing your photo.

 

[achris]

.. and determine whether or not these block off plates actually serve a purpose...

Yes they do... Here are some quick and dirty drawings I did to show you what will happen when you are running with and without the block off plates....

 

[soulsalvation]

Yes they do... Here are some quick and dirty drawings I did to show you what will happen when you are running with and without the block off plates....
View attachment 345753

thanks for responding. wouldn’t my manifolds get hot? they were the coolest i have ever felt them after 10 minutes at 1200 rpm. i appreciate your drawing but i still think a careful trial run to see what happens. thanks again.

 

[achris]

The manifolds aren't solid, they're hollow and it'll take more than 10 minutes for the heat to radiate to the outer part of the casting...

Good luck with the trial. I wouldn't risk it through. The likelihood of a crack in the internal part of the casting is high, and you won't know until you end up with either a hydrolocked engine or water in the engine oil...

Chris...

 

[soulsalvation]

Okay. I’m still wondering! Take a look at this schematic carefully. It’s still very baffling. It almost looks like those plates removed would bring more water through the manifolds.

 

[soulsalvation]

Here’s another one Chris. Thanks again for your feedback. Maybe a call to Fond du Lac in Wisconsin May clear things up. That plate almost seems to be a modification of sorts and not the original design. My manifolds have circulation channels that are blocked off by those plates.

 

[soulsalvation]

Do think if I turn on the engine for a few seconds to see if water makes it’s way through the manifolds to the exit tubes at the front of the manifolds that go back to the thermostat. Yesterday I could feel warm water making its way through but how much I couldn’t determine