high speed overheat

[fredrk2]

My Johnson 90 HP runs great, but the overheat horn goes off after about 15 seconds at full throttle (5000-5200 rpm) with good water pressure (24 psi). I have a new water pump and new spark plugs. What could be the possible causes of this? Thank you!

[Solarman]

Many times overheating at high RPMs can indicate (but not always) that exhaust pressures could be interfering with water pump pulling water.
For example, the aluminum webs in the lower unit, and midsection, are separate channels. The exhaust channel goes down through to exit the prop (most cases), but if those mating surfaces (where lower unit matches up with midsection) are deteriorating, then high pressure exhaust can leak over into water pump channel, and can reduce the flow of water being pulled in. sometimes they have separating grommets in the exhaust channel, they dry rot, get overheated if water pump goes out, and if not replaced, can cause same thing.
On my motors, I have a sub-water pickup for high speeds, and that tubing runs through the exhaust chamber. I have grommets that seal this tubing, but if water pump goes weak, lower water flow causes these grommets to burn out.

Another possibility for your motor, the grommets on the water tube that goes to supply the powerhead may be cracking/ not sealing, and allowing some water pressure to leak out, reducing water flow through motor.

And then, another possibility, you might have some blockage in some passages internally, causing those areas to overheat at higher RPMs. If you use a corrosion flush (vinegar, salt away, or similar chemical) for an hour or more (in large container foot fits into), (in stages, 15+ minutes run, then an hour to allow solution to cool, and another 15+ mins run- till solution hot again, then let cool again), this may break down the salt corrosion inside. It would be helpful to buy a laser temp gun (~$15 at harbor freight), and while running motor on muffs, aim at different areas of cylinders to see where temps are increasing the most. If at thermostat, you may need to remove and clean or replace, it may not be opening completely?

Internal blockages do occur even if religiously flushing motors. It just happens. If you catch it in the earliest stages, a chemical flush can help. The laser temp gun can tell you where your motor is getting hottest. During flushing, this temp gun can tell you if those areas that got hot are opening up and cooling down.

If badly blocked, it may require opening those areas up and cleaning manually?

Good luck, post back if you find out anything.

[fredrk2]

Thanks for all the great info! I'll let you know the outcome.