Buddy of mine has a Merc Bravo 1 454 in his eliminator boat. It's a IO with the exhaust that goes through down to the prop. Last time he was out he killed the raw water pump and it caused the motor to get pretty hot. From what he told me he was running 55 (maxed out) and then suddently it was running 50, and then suddenly 45... he then looked and realized the temp guage was pinned (240) and shut it down. He brought it by my house i found that the raw water pump impeller gernaded and blocked any water from getting by. After we fixed that we changed the oil and when i pulled the plug i found a good amount of metal shavings on the plug (it's magnetic) but the oil felt clean and i found nothing in it. THe oil was black, pretty thick feeling to me anyway. Got new merc/quicksilver oil in there and started it up, it seamed to run ok so i was thinking he caught it before any real damange happened. Now a couple weeks later he started it on the trailer, hasnt ran it since we did all this work, and there's a ton of black suit looking shit comming out of the exhaust with the water, it cleans up but if he gets on the throttle looks like more comes out plus it's smoky, the smoke is a grey/black color, doesnt look blue, could possibly be white but that could be steam... Wondering if this is related and if this thing is going to let loose on him when he takes it out in a week. Have not done a comp test on it or leak down. He doesnt seam worried at all about the motor, but i'm kinda anal about this stuff and now that all that black shit is comming out it's making me wonder....
Lost 10mph from overheating ? Never guess everything is fine.
Engine: check compression.
Exhaust - exhaust needs water to cool everything down related to the exhaust. With thru prop exhaust, there is a flapper valve down near bottom of y-pipe that can dislodge or burn up from being overheated. This then could lodge itself and create a major blockage to exhaust flow! Maybe cause of mph lost if not the engine itself ?
Also rubber exhaust hose connectors and etc can be subject to burn thru.
Lastly, the peieces of impellar went somewhere ! Typically they will be all stuck at water inlet to oil cooler. If not, you need to go further downstream and find those pieces !
The impeller broke up into pretty large peices, looked like each flat just broke right off, i cleared out what i could from the lines while i had the t-stat housing off becuase that was what we checked/replaced first incase it was getting stuck.