Exhaust riser restrictor plate - Volvo Penta sterndrive 3.0 GLP-D 4-cylinder petrol

[Kwanza]

Hi all, looking for answers to the below issue affecting my first ever boat, recent owner.

Volvo Penta 3.0 GLP-D, 4-cylinder, sterndrive, petrol engine on a Glastron 5.2 m fiberglass boat.
Engine runs OK, warming up to ~175 F in ~10 minutes, and will run at this temperature.

Top J-hose, from engine circulation pump to thermostat housing becomes stiff, like a tennis ball. Occasionally, this hose would split due to the pressure.
I'll do a 40 km trip with the recommended 3200 RPM, all good, then next day it will split after 10 minutes. No overheating, still ~175 F, only pressure.
The hose on the other side of the thermostat, going to the manifold is of a thicker material and smaller diameter, difficult to gauge how pressurized it is.
If I remove this hose from thermostat to manifold, then the J hose from engine circulation pump to thermostat is not pressurized any more. As soon as I replace the thermostat to manifold hose, the J hose from engine circulation pump to thermostat is again pressurized.
I have an exhaust riser extension - manifold + riser extension + elbow. Both the extension and elbow have four slots for the water circulation, however there is a restrictor plate on top of the extension, that is, between the extension and elbow.
All slots with fair bit of residue & clogging. Based on previous owner's service receipts, manifold and riser were replaced 3 years ago.
The position of the slot on the restrictor plate is on the side closest to the engine.

What are some of the paths I should take to try to solve this issue:
1. Replace manifold and riser as it is, with restrictor plate.
2. Clean riser extension and elbow and try to run engine without restrictor plate - observe operating temperature.
3. Clean riser extension and elbow and try to run engine with restrictor plate

Thank you for your help, and apologies if something doesn't make sense in my description.

 

[spannerman]

Well you definitely have some kind if restriction in the exhaust. I would clean all the passages and check that the water entering the exhaust bend can actually escape out of the other end. Then refit with new gaskets.

 

[Kwanza]

Thank you. In addition to the previous information, I note that the J hose in question (shown in the video), between engine circulation pump and thermostat housing is quite warm, similar to engine temperature after warm up, ~170 F.
A cooling system diagram I came across seems to indicate this J hose should be cool, if I read the below diagram correctly. Should this be the case, what are the possible issues causing this J hose to be hot (and pressurized) that might not be a cooling water flow restriction? Head gasket issues?
Thank you.

 

[spannerman]

I think the reason the J hose gets warm is because the water isn’t flowing as it should. You say the hose isn’t pressurised if you disconnect the hose to the manifold which confirms the restriction is downstream of this. Have you checked that the elbow fitting where the hose connects to the manifold is clear, are the exit holes in the exhaust bend clear as these tend to rust and restrict the exit flow.
These GM motors are very under stressed and are solid cast iron and seldom have head gasket problems, but to be sure you could fit a clear hose between the thermostat housing and manifold and look to see if there are any air bubbles in the water when the engine is warm.
Does the manifold or exhaust bend get hot? you should be able to place your hand on them without it burning which indicates a good through flow of water. If there is a restriction the water stays in the engine too long increasing the temp.