If you haven't cleaned the heat exchanger, that's a likely candidate. I ran one of these engines for 22 years of it's 32 year life. Just replaced it last year because I couldn't get parts
for trans rebuild
. Seawater cicruit on mine was routed from seacock/strainer to trans to pump to heat exchanger to exhaust elbow. If you've replaced the elbow, and cleaned out the trans cooling passages, that leaves the heat exchanger. Mine was an old aftermarket Sendure unit, but most on that engine are similar.
If the coolant passage
in the trans was blocked, most likely the heat exchanger is, too. It will also collect impeller blades and pieces of zinc, which will shut the flow down to a trickle. If it has an end cap that you can get to easily in place, take it off and see what's there. You may solve your problem. If not, don't mess with trying to clean it it place.
Just take it off and take it to a radiator shop. They'll clean it up for minimal cost. If that's not convenient, I cleaned mine in the back of beyond one time by removing the end cap, standing it up in a bucket, capping the seawater outlets, and filling the seawater side with a solution of 1/3 muriatic acid, 2/3 water. It foamed for about 20 minutes, and it was clean. If you do this, wear rubber gloves and be careful. Vinegar takes way too long. Muriatic acid is easy to find and cheap -- any paint
store or hardware
store will have it.
If the engine has always been freshwater cooled, there's not likely to be much scale in the block, unless somebody didn't bother with coolant
for rust inhibitors. You can judge that by looking at the fresh water passages in the heat exchanger while you're cleaning
it. If that's cruddy, once you put the heat exchanger back, run some caustic cooling system flush through the fresh water side. Get it from an auto parts store, and follow the directions.
The first time I did this job, I replumbed everthing so that the trans was freshwater cooled, as well, eliminating one source of rust.
Good luck. Loved that old engine, but I'm happy with my new Yanmar
. Happy to provide more detail if it's useful.