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Discussion Starter · #1 ·
I have a Yanmar 3GM30F diesel engine. It is fresh water cooled with a heat exchanger.

I have invested several days now trying to figure out why when the engine is turned on and run (up to 2-3 min.) no water comes out of the stern scooper (exhaust). When the engine starts I have seen exhaust come out (white and once a bit black and then white) and see air creating ripples in the water, but no water is coming out - not even a drop.

I have cleaned the sea water filter, checked the sea water pump impeller, cleaned the 24 tubes in the heat exchanger, and released water from the little screw in the muffler. After disconnecting, cleaning, checking and reconnecting all of the above, I verified today that water is circulating up to the mixing elbow.

I have read that corrosion can build up in the mixing elbow. It is screwed on very tightly and I do not have the right size tool to remove it thus far and that's why I haven't been able to check it. Could there be an issue with the muffler? Tomorrow I will check the hose that leads out to the stern scooper, but I have not observed an increase of water in the bilge.

Any suggestions or comments from someone who has had a similar experience is more than welcomed.
 

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Possibilities: Slab rust in exhaust manifold blocking the water flow or totally blocked injection elbow.

remove the water exit hose from the exhaust manifold and 'blow' into it. If no air/mouth flow or very high restricted flow then you have a blocked injection elbow nozzle. If so then remove the injection elbow and either clean it out or replace it.

remove water inlet hose to exhaust manifold with exit hose hose also removed. Place a suitable length of new hose onto the inlet to exhaust manifold and 'blow'. If no water comes out of the exhaust manifold or there is a very restricted flow then the manifold has developed slab rust and one or more of the loosened slabs has blocked the water outlet from the manifold internals. If so you can remove the manifold and insert a stiff steel wire to break up the slabs. Usually when a manifold begins to show 'slab rust' it can also easily develop gas side to water side pin hole leaks and should normally be replaced. To prevent slab rust, NEVER ever allow the raw water cooling circuit to 'dry out' for long term storage, rather always fill with a mixture of 50:50 ethylene glycol (with rust inhibitors) and fill the exhaust manifold/engine, etc. Slab rust (destructive ferric -red rust) will rapidly form in an engine that is run at below normal operating temps (~180 deg. F) ... running an engine sufficiently at 180°F will promote a 'protective' form of rust - blue-black rust or ferrous rust. Run hell out of a marine engine (to 'heat soak' it) to prevent and retard internal ferrous corrosion.
 

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If it were my motor, I would pull the raw water hose at the exhaust injector elbow and run the motor momentarily to get an idea of the water flow and pressure that the raw water pump is developing. Good flow and pressure would rule out everything before that point. You might find that there are two problems. Restricted exhaust elbow after the injection fitting as RichH talks about and worn raw water pump cover plate and the other end, so that the water pump is too worn to over come the mixing elbow restriction.
 

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Discussion Starter · #4 ·
RichH and Ray, thanks for responding. The problem was indeed the mixing elbow. Turns out the hose leading to the muffler was not as strongly fused to the mixing elbow as I'd thought and I was able to carefully pry it loose with a screwdriver. I was able to remove both the exhaust elbow and the mixing elbow together by removing the four screws that attach the exhaust elbow to the exhaust manifold. I tried blowing into the mixing elbow and running water through it with the hose and noticed that it was completely blocked. I read up on mixing elbows and concluded that the best option was to replace it. However a friend stuck a flat, flexible wire through the mixing elbow and was able to remove enough of the corrosion to get a good water flow through it. I reattached everything and bingo! water flowed fine through the exhaust now. I know it is a temporary fix and will replace the mixing elbow as soon as possible. Thanks for the advice.
 
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