When running at cruising speeds my water temperature cycles from 160 to 180 or so and sometimes activates the warning alarm in the process. I never had this problem when running a restrictor in place of a thermostat. After adding an 8an bypass hose and thermostat the problem started.

I am running a 496 ci BBC. I am using the stock mercruiser raw water pump and the impeller is new. I am running a large thermostatically controlled oil cooler and all water hoses are plumbed as factory. Water then runs to my crossover (no circulating pump) that has an 8an bypass hose running to the top of my thermostat housing. I have used a 160 degree high flow stat and also a mercruiser 140 degree stat. The fluctuation is the same. The only thing I have not tried is a thermostat with holes drilled in it.

Thanks in advance OSO members!

I wonder what the chances ther is some fouling floating around in the block.....would it be a major hassle or is it possible to back flush the system and see what if anything comes out?