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Mercury 150 possibly 1989
serial # A933666

OK.... ran fine on this boat for the first two years on it.
it has new impeller, new poppet, new thermostats, pulled the block and checked adapter plate found some sand and cleaned and replaced with new gaskets. At idle i have about 4 psi.... at WOT i have about 18 psi... temps about 120 at idle ... 160 at 3000 rpm..... anything beyond that rpm and it climbs slowly towards 180 ... at WOT it heads towards 200 and i will not go beyond that... I remember it staying nice at 140 at WOT....
I just do not know where to look next..... it runs strong and will hit 5800 rpm easily... I get a sense it is a leaving water problem ... I am so frustrated.... Any one have this type of problem or a solution?
 

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Are the head gaskets blown and compression pushing the water out of the block at high rpm?
 

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I too have a black max, I don't think I am overheating, but I have noticed when I'm at higher rpms and running around 60 or so, heck even 50 the water stream almost looks like it's dribbling out, but when I'm creeping or idling it shoots out a solid pee stream. Thoughts???
 

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Bad thermostat or maybe the wrong one? Water pump arms turned in the wrong direction or installed upside down? I always fix things one thing at a time so if something goes wrong I could back track it. What was the last thing you messed with before it started acting up?
 

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I can tell you I had a buddy had that happen to him drove him crazy it came down to a new transducer he had installed and where he put it. The position of the transducer changed the water flow when trimmed out at top speed and caused the motor to over heat. So have you added anything to the transom look at that sometimes it's little unsuspecting things.
 

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If your impeller, poppet, and thermostats are good, look into pulling the power head and changing the gasket in your mid. You may have a 1 or 2 piece exhaust adapter plate. This is a very easy job just time consuming. My 225 PM was doing the same thing as you describe and took me 2 weeks to figure it out. The gasket was probably the original and was in very bad shape. Where it was torn would not allow pressure to build up in the block to open the poppet.
 
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