Yamaha F250 Overheat at 4800+ RPM

[hstechfl]

2008 Yamaha F250
F250BTXR
Serial 692-X-1030827
300 hours

Hello All,

First time posting on the Yamaha board, previously had an Optimax and got a lot of help on this board so here it goes..

Have a new to me 2008 Yamaha F-250. Runs fine idle and trolling, but when its up to plane the overheat alarm will sound if I keep it over 4800 RPMs for more than a couple minutes. The Yamaha gauges usually shows the meter in the middle position, but it will creep up 2-3 points when this is happening. I have an engine data connection through my chartplotter as well and the temp on this has shown an alarm at 175 degrees and even alarmed at 145 degrees..

I'm also hearing a weird sound from this motor. Its hard to describe. The only thing I can think of is a coffee maker when its finished brewing, trying to suck that last bit of water up the peculator. Only does this on plane also. And now yesterday, the speedo stopped working. I'm guessing this is a pitot device that is using water pressure to measure speed.

My feeling is that all these issues are related. If this was an Optimax- my guess would be the poppet valve since this is only happening at high speeds. Not sure about the plumbing on these motors, but it seems like water isnt going all the places it should be- maybe one of them being the pitot?

So far the thermostats and the impeller have been replaced- and the problem remains. Water tell tale looks weak- but I heard this is normal with Yamaha. It does increase as RPMs increase.

Any suggestions would be appreciated! Thanks

[shallowhal]

Im not sure on the F250 but HPDI's are known for crud stuck in the Poppet valve. Ive done mine 4 or 5 times now, its not too bad once youve done it a couple times. I think there is a post pinned at the top to show you where it is, otherwise google has a bunch of pictures. Usually sand gets stuck in there keeping it open and not allowing pressure to build. once you clean it out it seats better and gets you the pressure you need.

[hstechfl]

Thanks- from what I have searched, the poppet is on the starboard side behind the cover above the tell tale?

[shallowhal]

If its the same as the HPDI, yes. And its under there, so be prepared to be frustrated.

[hstechfl]

If its the same as the HPDI, yes. And its under there, so be prepared to be frustrated.

Thanks- I'm always prepared

[hstechfl]

So I checked out the poppet valve- and everything looks OK. Other than a little salt around the o-ring, it looked new. Cleaned it off and reinstalled.

I'm starting to think my transducer is causing the issue. Its placement may be allowing air to be sucked into the low water pickup. Could this be possible even if the engine is 'peeing' the whole time?

[CJ1]

Yes, something causing cavitation by the intake screens. On a hpdi or sho the screen being a little out of place at higher speed will cause this issue. Engine up too high, what is your water pressure? Not familiar with the water intake on a f series. Cj

[Ranger519VS]

Yes, something causing cavitation by the intake screens. On a hpdi or sho the screen being a little out of place at higher speed will cause this issue. Engine up too high, what is your water pressure? Not familiar with the water intake on a f series. Cj

Water intake is the same as the HPDI. It seems I cannot run mine high enough to lose water pressure. the poppet valve should be fully open on any on plane speeds or it could overheat. Could be the transducer flow thing.

[Texas Larry]

The warmest I have ever seen my HPDI is 135, and normally runs between 127 and 131. What is your water pressure at 4800 rpm? My understanding is that the poppet valve is a pressure relief valve that only opens until the engine warms up and closes when the thermostats open. A weak poppet valve spring would allow the poppet to open when it should be closed allowing water to bypass the engine water jackets and could cause your problem - i.e. the poppet valve partially open when the engine is running at higher rpm. The little rubber grommet that the poppet seals against could also be deteriorated. The poppet valve parts that are being sold now are an upgraded version of the originals in the early engines, if I remember correctly.