Re: 40 horse Merc outboard overheats
That's a strange one!<br /><br />If it were mine, I'd drop the lower unit and check out the water pump. You changed the impeller, but maybe the cup, plate or housing is at fault. Any grooves worn in the cup or plate will affect pump performance. Is the impeller installed correctly? ie, are the blades facing the right way and is the key installed correctly? Is the grommet in place where the water tube fits into the housing?<br /><br />It sounds as if the pump is weak for whatever reason, and the turbulence caused by the prop turning is enough to stop it pumping. You could remove the prop and try running it in gear to eliminate this, but it's probably a good idea to replace the water pump anyway.<br /><br />Also, while the lower unit is off, I'd attach a hose to the water tube, remove the small hose that leads to the peehole, and flush the system out under pressure, just to be sure there isn't a small blockage anywhere.<br /><br />Good luck.<br />Steve
