Yamaha HPDI Overheating

[1d10t]

2001 model.

150 HP

Overheats after 30 mins of 4600 rpm. Replaced pump. Checked thermostats, sensor, and valve.

What else could be the issue?

[Ken]

1d10t,
When you say overheats, what do you mean?
Alarm going off?
Motor going into rpm reduction?
Do you have a temp gauge? How about a water pressure gauge?

In general, overheating can be caused by:
1)weak water pump - a good water pump will give you about 5 psi at idle and 20 at 3000+ rpm.
2)stuck (closed) thermostat(s), or clogged stats.
The thermostat is designed to be fully open at 140 degrees f. - approx 0.120 inches. You can test in a pan of heated water and thermometer.
The stats can get clogged with debris/mineral deposits and not open fully. Remove, inspect/clean.
3)water pump intake obstructed by debris
4)pressure relief valve stuck open and dumping cooling water - the prv is designed to open at water jacket pressure of 20 psi and more. The prv can get clogged with debris and not close fully for pressures less than 20 - there will be water dumping out of the relief holes at the lower rpms.
5)clogged water jackets - mineral buildup. Heads have to be removed and the buildup cleaned out.

If your motor is indeed overheating, I would guess #5 above is the most likely suspect since you are able to run 30 minutes at high rpm before overheat occurs.
You can pull the thermostats and look into the water jackets a little to see mineral buildup.

If you don't think the motor is actually overheating, you could be getting a false alarm from a faulty thermo switch. There is one in each head. You can test the thermo switch in a pan of heated water and ohm meter. The switch is designed to close at around 180 degrees f. and re-open at approx 160. I have personally had one close at 130 and give me a false overheat alarm and rpm reduction.

Good luck, and let us know what you find out [img]smile.gif[/img] .
Ken K