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5.7 Gi overheat issue

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  • 5.7 Gi overheat issue

    Hey guys I've got a weird overheat issue. The engine is a 5.7 Gi-C with 450ish hours on it. This boat is a sea water cooled boat and runs on freshwater lakes. The boat can idle all day without overheating and most of the time it doesn't overheat at WOT. My issue is going from running then down to idle it will start climbing temp. If I put it in neutral and hold it at 2k rpms it will cool back down. It goes hot but once it drops it will drop down rapidly and be under operating temp maybe 120-140*.

    So far I have removed the impeller and inspected, looks fine. It is aftermarket but looks to be a good copy, I am ordering a OEM one. I replaced the thermostat, although when testing the old thermostat it operated correctly. I also drilled a 1/8" bypass hole in the stat to bleed air out easier.

    I removed the exhaust risers and they look fine. I had rust sediment in the engine so I removed the thermostat and poured a diluted solution of muriatic acid in the engine to dissolve the rust. It looks to of cleaned it up quite a bit but still same issue.

    I am at a loss on this. I haven't removed the circulating pump yet but I don't think it's that since its not always overheating and I can feel the water hose from the pump get warm when running. I am leaning towards a head gasket and it is pushing a large slug of combustion gasses in the coolant path.

    It's not too hard to pull a head on a chevy but I don't want to if this doesn't seem right. What are your thoughts on this?