I talked to a marine heat exchanger specialist http://www.holtcorp.com/
just last week about this issue.
He recommended:
Check the raw water intake side of the oil cooler, transmission cooler and
heat exchanger for grass, seaweed etc.
(I disassembled the exchanger and reamed out a few of the holes with a
straight piece of coathanger, then I poured water into the top to make sure
it flowed out the bottom.)
Check all hoses for kinks.
Drop the coolant mixture to 20/80 from the 50/50 (Mercruiser only) I'll
drain all of it for winter.
Check the flow of raw water out of the exhaust pipe at 1000 rpm's. I don't
have the expected flow rate with me.Give the amount that ends up in a 5
gallon bucket after 15 seconds and I'll reply to the group.

I removed the water flow sensor as the 1" pipe is reduced by about 50% by
the plastic valve inside.
I don't know which one worked but I'm not overheating at high ( 3800) rpm's
anymore.






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