3.0 exhaust manifold over heats

OK this is a diagram for a 1998 OMC Cobra 3.0 but keep in mind this is the joint venture period when OMCs and Volvos were basically the same. So there is no bypass to the exhaust pipe here and I think the exhaust manifold and elbow get a constant flow of cooling water. If the OP can convert his engine to this system and use a 160 stat, it may solve the problems. I am still surprised that Volvo designed it that way with no bypass of cool water to the exhaust elbow which is always the hottest part of the system. If the 'stat doesn't open fast enough, that will cause a very rapid increase in temp with the manifold right up against the side of the cylinder head, all that heat gets transferred to the cyl head.
So the OP might have to source a different 'stat housing and block off that port for the bypass on the exhaust pipe to prevent water leaks. Not sure if it would work, but what do others think?

Lou C said:

That would actually "bypass" the elbow and the water goes straight into the exhaust pipe!

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Ah, I see. You are right of course! I was looking only for water downstream of the manifold and saw that. No coffee yet is my humble but not worthy excuse. :-(

oops...

I never thought Volvo was better than Mercruiser. Cost difference was a snob appeal thing.

Thanks for all the input here since last time.

I have now changed the head gasket, and also went through the valves and seats while I had the head of. Now it’s back on, I still have to set the rocker arms to the right tension though.

There was a question from Lou C about the bypass and where it’s connected.

The bypass is connected to the elbow right after the manifold. Does not match the diagram in my first post but this is how its connected. Pictures of engine attached.

I also noticed I was missing the restrictor plate between manifold and elbow. Found it in the boat, maybe previous owner forgot to put it in after changing the manifold. But I can not see how a restrictor plate can improve cooling of that manifold, think it might do the opposite. Anyway, I have put it in now to give it a try.

The new thermostat is a 160.

I have looked around some on the internet, and found out that there another type of thermostat house that will route the bypass water out that same way as on open thermostat will do. This one will provide constant flow of cold water to the manifold.

OK what you show there makes sense, the diagram did not, because the elbow would not get cool water flow till the 'stat opened, and it would run VERY hot.

One thing I have been thinking of during all this time. What if I would switch the hoses from the thermostat house. Routing constant flow of bypass water through the manifold. And the warm water from when open thermostat to the elbow.

I also noticed I was missing the restrictor plate between manifold and elbow. Found it in the boat, maybe previous owner forgot to put it in after changing the manifold. But I can not see how a restrictor plate can improve cooling of that manifold, think it might do the opposite. Anyway, I have put it in now to give it a try.

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Ayuh,...... Without the restriction, the water flows to fast to draw out the heat,......
Put it in-place, as that's yer problem,......
'n don't change anything else,.....