Water Pressure Drop at 4000 RPM's - Overheat

[jallen21750]

I have a 2007 Yamaha HPDI 250 2 stroke outboard that quickly loses water pressure at around 4000 RPM's.

Maintenance and Observations:

1. New water pump kit with housing, etc.
2. New thermostats
3. Tell Tale stream is strong at all RPM's
4. Sufficient water pressure from point of acceleration to 4000 (15-25 psi)
5. At around 4000 RPM's, all water pressure is lost (0 psi)
6. After approximately 10 minutes, the overheat alarm will sound

I have taken it to two mechanics and neither one has been able to diagnose the problem.

Any ideas?

[Bilge Rat]

Check the poppet valve.
Is there anything on the hull causing cavitation?
Check the inlet screens on the LU, are they installed properly?

Motor mounting height?

To have good pressure then lose it at speed with forward motion pushing water into the inlet is sometimes caused by outside conditions. Transducer or livewell pickup, motor height, bad hull repair etc.

Eliminate outside sources as well as internal motor problems.

[boscoe99]

I have seen something very similar once. Water pressure would be fine until we hit the magic RPM/boat speed at which time the water pressure would be lost, the RPM reduction mode would kick in and the buzzer sounded.

Found the inlet screens to be projecting outward from the gear case by about 1/32nds of an inch. Took the inlets out and sanded the inner surface down by or about 1/32nds of an inch. To the point where the inlets were flush with the gear case. Motor ran fine thereafter at all RPM's and boat speeds. Brand new motor by the way.

Has anything changed with respect to the gear case or the bottom of the boat?

Oh, sometimes water pressure can be maintained or recovered by raising the motor height if a jack plate is installed. Just contrary to what one would expect.