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What you do is measure the temperature of it with an IR temp gun after the boat has had a good run. As they clog up, they no longer properly cool. On a diesel engine, you do not want to see anything any hotter than about 180F. If that elbow is 180F or hotter, it needs to come off and be inspected, which really means replace it. But if temps are under the 180F, then it is cooling properly, but it would still be a good idea to pull it and inspect the surfaces that the gasket sits on. As the elbow rots, the surface area gets thinner and thinner. Eventually it rots away completely and seawater will dump down the exhaust and into the engine. What you want to see is a good solid 1/4 inch or thicker on the gasket surface areas. If its any thinner than that, the elbow should be replaced.
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decided to replace elbows what should this cost