The aftercooler should be taken apart and cleaned. Yes, the fresh water will help but that means instead of the every 2 years in salt water they should be done every 4 years, maybe a fifth but the first time generally at 4 and then think about extending the interval based on condition observation.
What happens is where the cooling tube bundle cartridge joins the housing there will be some corrosion, fresh or salt. If it is let go long enough the corrosion will freeze the tube bundle in the housing.
If unlucky the tube bundle can be wrecked by poor removal techniques OR being to late.
If stuck and If really lucky it can be removed by long soaking with a good penetrating oil such as Kroil. Maybe some gentle heating.
SOmetimes the penetrant must be poured into the housing through the air holes so it acts inside out. Then turned over so the other end gets treated/soaked also.
GENTLE tapping may get it to move a bit at a time. Do not just keep hammering in one direction but reversing the direction to slowly break the corrosion up. Use a thick block of wood, built up plywood or something as the tube ends will not withstand direct hammer blows. The block needs to almost entirely cover the tube bundle end. Lots of relatively gentle tapping is better than a few heavy blows.
Once out then the bevel on the housing must be inspected. Any pitting must be corrected. Often entails some light machining. One fellow here used a router with the appropriate 45o cutter and it worked. He only needed a light clean up. Failure to do this will result in seawater misting of the engine
leading to serious damage. Those pits will allow water to get past the O rings into the air stream.
Reassembly will require some heavy waterproof grease such as ALCO METALUBE and lots of it. on the various sealing surfaces.
RECOMMEND: Or you can go to Seaboard Marine,
www.sbmar.com , and look up Tony Athens' article and get more tips.
Alco Metalube and Kroil are available from Seaboard.
ANother plug. GO to Seaboard and do some serious time reading. He is a guru/expert on the Cummins engines, 6BT and 8.3 among others. What makes him terrific is on his site he has offered a huge amount of info for free. You don't have to do anything except read and learn. Then use what he has offered as you see fit. He is not the only one to be extremely helpfull but his articles have made the info available to anyone who looks.
And by the way although he has written the info for the Cummins engines most of it also serves any other engine.
As for the coupler rusting, that can often be simply inattention to the stuffing box. A simple old beat up fender can be cut to cover the stuffing box shaft entry. Those stuffing boxes will throw a fine spray around a wide area which will cause rusting.
