I've got a pair of 2005 300 HPDI's on a 31 Contender that were updated 140 hours ago. Total time now is 240 hours. A couple of weeks ago I was just 10 miles into a 75 mile return from the stream when the port motor started shaking and skipping {but not severely}. Temps were cool,but the motor would only do 4100 wot. We settled on 3800 and came on in. <br /><br />We decided the problem had to be a fouled plug or two since we had stayed [for the first time ever] in just one place trolling for 12 hours.Sure enough there was right much flaky black carbon on some plugs. I brought the boat home and decarbed both engines and changed water pumps as routine maintenance. <br /><br />When the lower unit to the port motor [the one that was running rough] was dropped,I found some hen egg sized chunks of carbon-laden melted aluminum 
. Upon looking up in the upper case, I could see the bottom end of part of the exhaust manifold missing. I hooked up the YDS to the engine and it showed everything normal, with no overheats.<br /><br />Now the question....Did the exhaust manifold burn off or was it a bad casting? At any rate when it went to the bottom of the lower unit it looked melted. If it did burn off what has happened since the motor has never overheated? Do you think the exhaust manifold problem lead to the skipping [by perhaps detuning it]. Thanks. And sorry for the long first post.
. Upon looking up in the upper case, I could see the bottom end of part of the exhaust manifold missing. I hooked up the YDS to the engine and it showed everything normal, with no overheats.<br /><br />Now the question....Did the exhaust manifold burn off or was it a bad casting? At any rate when it went to the bottom of the lower unit it looked melted. If it did burn off what has happened since the motor has never overheated? Do you think the exhaust manifold problem lead to the skipping [by perhaps detuning it]. Thanks. And sorry for the long first post.