Hi Pat,

It sounds as though you are on the right track.

The ends should be sealed with a rubber o-ring. Depending on the engine type there are sometimes 2 o-rings. There should not be silicone. You should be able to look through the tubes and see nothing but air. Any obstruction is going to cause you grief especially near the ends as this is where the sea-water changes direction. If you look at the design of the HE, you will see the how the seawater changes direction two times before moving on.

While you are in there, also check the after-cooler, if you have a turbo charger. This can be another are where the copper tubes become blocked and foul the flow of sea-water.

The anti freeze is moved along the outside of the tubes by the engine’s fresh water pump. The sea water is moved by the sea-water pump impeller on the inside of the tubes. The anti-freeze is cooled as it makes contact with the other side of the copper tube.

Hope this helps you.