Since not everybody on the planet has your
engine, a picture of your
heat exchanger would be helpful.
For my
Yanmar, the tube stack in the heat exchanger is accessible by removing the end caps, which are bolted on with two machine screws on each side.
Yes, drain the
coolant and sea
water from the heat exchanger before removing the end caps. Its drain cocks should be obvious.
Follow the hose and pipe from the
water pump">raw water pump to the heat exchanger to find the inlet side and remove that cap first. Any large impeller pieces will be stuck there. If you want to look thru the tube stack pipes then you have to remove the other end cap. If you do that, then remove the tube stack and clean it regardless if there are any impeller pieces stuck in the tubes. The tube stack should be removed and cleaned every several years for no other reason than to keep it from getting stuck in the heat exchanger and then it is a b!tch to remove when you *really* need to remove it.
If you are the proactive sort, then
purchase the o-ring/gasket set for the heat exchanger end caps before removing them.