Disconnect the hose from the water
pump to the engine on the discharge side and disconnect the hose from the discharge side of the heat exchanger
that connects to the injection nozzle. Make a temporary hose connection to your fresh water source to the hose "from" the heat exchanger
; and, a temporary hose connection to the hose on the discharge side of the raw water pump
(to the engine) which will be the discharge from the back-flow exercise. Direct that to a bucket to capture the back flow. Start your back-flow "gently" keeping in mind that shoe side water may be at 40PSI or better. Flush until the discharge into the bucket seems to run clear.
Once that's done, make up a solution of Barnacle Buster and fresh water in your bucket and attach the discharge hose from a small submersible bilge pump
, submerged in the bucket solution, to the discharge line from the heat exchanger in place of the fresh water hose from shore side. You can power the pump
with alligator clips temporarily connected to your batteries
. With that, let the solution "back flow" through your heat exchanger for a couple of hours. You may be astonished at the amount of gunk that will be released. (Put the pump
in the foot of a woman's nylon stocking to "filter" the solution going back into the heat exchanger.)
Once that's done, reflush the system with fresh water and then restore the original connections and you're good to go.