cooling problem

[alablount]

I have 99 DT115 just replaced waterpump impeller kit,exhaust gasket and all
other seals related lower end.Works great at low speeds but overheats at high speeds.Could water pressure valve cause this problem? If so how can I replace
it.

[BillCon]

It, could, but also look to see if the motor isnt running to far out of the water at high speeds mine will do that if I, trim up to high and run my trim tabs down it will not get enough water, and also check the intake part of the foot take out the screens too

[Solarman]

dt115, overheats at higher rpms

I've had same problem with both my motors.
If new impeller, and works fine at idle, but pee stream starts spitting and sputtering above About 2500 RPMs? Then what is happening is the exhaust is pushing Water back before it can get to pump. The seals between the water pump passage and the exhaust have a grommet seal between them located under the water pump. You will have to remove the gear case bearing cap to replace the grommet. The grommet seals the sub-water pick up pipe that helps feed the water pump at higher speeds. You should replace the grommets at both ends of the sub-water pipe. When removing (remember exactly how it comes apart) the bearing cap be careful not to drop any parts, and keep the parts clean (cover the hole and parts with clean rags). It might be wise to get a new o ring seal for the bearing housing when you get the grommets. The stainless plate under the water pump has to be put back in properly. That plate protects the rubber grommet from the high heat of the exhaust. My plates were in backwards.
If there is any corrosion, or areas where aluminum has been eaten away in the webbing where the gear case (foot) matches the midsection, those areas need to be repaired too. Those webs are not just for strength, they separate the hot exhaust channel and water channel.
Good luck and let us know how it's going.