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What would cause this,a leak? I did have a belt fail (delaminate) it slipped, on the raw water pump.

Poor water flow. Mine did the same on my 250/540's with a crossover. Removed the T-stat for now and it is better

Thanks I will try thermostat trick.

Pull the hose and look inside the header. The passages can get restricted with junk. Use a coat hanger to pick it out/through

do you have sea strainer like above said junk or a restriction or not enough volume of water / flow

just worked on exact same issue, oil cooler partially clogged

You should pull the inlet end off of your oil cooler at least once a year and run a rounded end brazing rod through each tube to clear any blockage. I have seen where half of the tubes were blocked.

Lack of water flow for sure. Check the Tstat housing where the check balls are for a restriction. That is what feeds water to that hose.
Also, keep in mind, those stock Merc hoses with the steel braid get bad internally even when the look good.

Well It has a brand new oil cooler..New circ pump,new sea/fuel pump,new t-stat.raw water no strainer

My new sea/fuel pump may be suspect due to the fact I reused the old gaskets and body(2 piece) with a new impeller.

Water temp gauge never went above155. am going to check these suggestions along with the replacement of pump impeller and housing.

Pulled the sea pumps on both engines impeller housings worn out.Should of replaced them before.

New ones now should be good to go.