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Author Topic:   Yamaha Ox66 225 Overheat Alarm
onlyawhaler posted 07-22-2015 02:37 AM ET (US)   Profile for onlyawhaler   Send Email to onlyawhaler  
I would appreciate some feedback from the Yamaha experts here. I have an 2000 Yamaha 225 Ox66 on my Outrage 20. It has always been a great engine, and I do the routine maintance on it, including cleaning the O2 sensor annually, draining the VST chamber for winter storage, plugs, etc.

Late last fall I launched with a few friends up on a lake here in Utah in colder weather. After 500 yards out of the harbor, under acceleration, I had a first ever, [overheat] alarm occur. I backed down immediately and turned off the engine. I used the kicker to come back to the harbor and trailer.

Since it was October and weather turned , I just did the usual winterization and put it away until Spring. I haven't gotten it out until a few weeks ago, and I immediately replaced the water pump, which I did only a couple of years ago. I did a complete water pump replacement with the housing and gaskets, not just an impeller. The impeller I removed looked fine. I also replaced the two thermostats on top of the block.

I took it out today for the first time this year, confident I had solved the problem. After five minutes of operation above 4,000-RPM, [the overheat alarm] came on again. I shut it down, waited 10 minutes and restarted and ran it back to the dock on a lower RPM, and the engine did not overheat again.

I use this boat in fresh water only, the intake on the gear case is always clear, and I am perplexed as to why this is occurring. First time this engine has ever let me down.

I have not raised the engine height or done anything differently that I can think of that is causing this overheat alarm

I hope for feedback that could help me troubleshoot this further. Thanks in advance


jimh posted 07-22-2015 08:53 AM ET (US)     Profile for jimh  Send Email to jimh     
Are you certain the alarm that is sounding is indicative of overheating?

Check the engine temperature. If the engine does not have a temperature gauge, perhaps an infrared thermometer could be used to get readings. That would confirm the overheating.

Have you flushed the cooling system thoroughly by using a hose adaptor or a flushing port? Perhaps there is an obstruction stuck in a cooling passage.

onlyawhaler posted 07-22-2015 10:45 AM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
Thanks Jimh

The Yamaha digital gauges have a LED thermostat that comes up when overheated along with an audible alarm. I had never seen that particular digital display come on until these last two outings. I have gone online to confirm that display.

The OX66 surprisingly does not have a flushing port that I am aware of that we see today on most outboards. Other than attaching muffs on the gear case and running it that way, I don't know of any other way to attach a hose to flush out the block. Any feedback on that being correct would be appreciated.

The stream of water showing circulation in the block is a tiny port, not set up for flushing.


onlyawhaler posted 07-22-2015 10:57 AM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
After posting that last comment about the OX66 not having a flushing port, I thought, that just can't be.

It does. Its not obvious like the E-tec,in the rear and combined with the pee hole .

Under the cowing towards the front there is a hose connector screwed in. I have never flushed the engine as it as always been in fresh water and I have never given it a thought. I will hook it up according to the manual and give it a flush. Hopefully that will help


contender posted 07-22-2015 04:19 PM ET (US)     Profile for contender  Send Email to contender     
Change your thermostats on top of each of your cylinders, cost about 25-30 dollars each and 3 dollars each for the gaskets. Use Yamaha parts...
jimh posted 07-22-2015 11:07 PM ET (US)     Profile for jimh  Send Email to jimh     
I think CONTENDER has given good advice. A thermostat that was stuck and not opening would lead to overheating.
jimh posted 07-23-2015 09:41 AM ET (US)     Profile for jimh  Send Email to jimh     
The cooling system of the typical outboard engine in the 225-HP range usually has two elements that control the circulation of the cooling water: thermostat valves and pressure valves.

The thermostat valves usually remain closed until the temperature of the cooling water increases to a certain point, typically around 170 to 180-degrees-F. Their purpose is to permit the engine temperature to more quickly rise to normal operating temperature upon initial start-up of the engine. The action of the thermostat valves will tend to control the engine temperature when the engine speed is confined to the lower speed ranges.

The pressure valves usually remain closed until the water pressure in the cooling system increases. An increase in water pressure is generally a sign that the engine speed has increased because the output of the engine water pump is very proportional to engine speed. With increased engine speed there will be increased water pressure. This should allow the pressure valve, sometimes called a pressure relief valve, to open. When the pressure relief valve opens, the cooling water flow is changed, and more cooling water is circulated through the engine power head. Sometimes there is a mechanical linkage between the throttle actuator and the pressure relief valve so that the throttle position also influences the operation of the valve.

In a V6 engine it is common to have a thermostat valve in each side of block, so there will be two thermostats. If either one or both become stuck there will be an effect on the engine temperature. If the thermostat is stuck closed, the engine temperature will tend to be too high at low speed operation. If the thermostat is stuck open, the engine temperature will tend to be too low at low speed operation and the engine may take a very long time to reach operating temperature range.

If the pressure relief valve is not opening or if the pressure in the cooling system is too low to operate the pressure relief valve, the engine temperature will tend to rise above the normal range during high speed operation.

Typically at high speed operation the engine temperature is not regulated very well, and the engine operating temperature will tend to be quite low, perhaps 100 to 130-degrees, depending the temperature of the cooling water. If the boat is operating in very warm tropical water, the engine may run much hotter, but it should not overheat. If operating in very cold water, perhaps 40 to 50-degree, the engine temperature at higher speed operation may be as low as 100-degrees.

Check the pressure relief valve and the thermostats if overheating occurs.

martyn1075 posted 07-23-2015 11:12 AM ET (US)     Profile for martyn1075  Send Email to martyn1075     
Its a good suggestion but I think he already had changed the thermostats already. At least that what was mentioned in the original post. Perhaps the new thermostat was replaced with another bad one? In that case I guess one could try that out.
martyn1075 posted 07-23-2015 11:24 AM ET (US)     Profile for martyn1075  Send Email to martyn1075     
Do these engines that with the VST also have any other sensors that could be faulty by giving out a possible phantom reading?

I have this same year engine but its a V6 200hp and its the straight carb version.

Although its a great engine also its less sophisticated then the VST model. I'm sure this engine has a few more sensors in regards to its control and I wouldn't be surprised if there is one hiding in there which has something to do with your problem.

Taking a manual heat reading of your block while the thermostats click in and out is a good idea because it bypass the digital Yamaha gauge. Perhaps the gauge has turned faulty.

onlyawhaler posted 07-23-2015 11:35 PM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
I have changed out the two thermosets along with a complete new water pump with the metal and plastic housing components two weeks ago. They were Yamaha parts. I just wanted to cover the bases before going back out. Still got the overhead alarm.

I thought that would do the fix, but evidently not. I have done the small fuel pumps and the VST chamber filter a year ago and haven't put 15 hours on it since then.

I am perplexed and the next try is just flushing out the power head as I have never done it before. I felt it was not needed because of fresh water use, but perhaps some sediment has built up over the years.

I will give it a hour long flush and take it out next week and report back .

Thanks for the feedback

onlyawhaler posted 07-24-2015 12:15 AM ET (US)     Profile for onlyawhaler  Send Email to onlyawhaler     
Jimhs post on the pressure relief valve got me investigating.

On my engine its called a poppet relief valve and I have never replaced it.

It could cause this situation even with good thermosets. I am going to order it and replace it as part of this process.


mikejana posted 07-30-2015 05:23 PM ET (US)     Profile for mikejana  Send Email to mikejana     
Not here very often anymore....
You have my greatest sympathy, as we share the same problem.
I have a 2001 OX66 250 mounted on a bracket behind our 22 Outrage. I split a pair of OX66s that CRBenny found here on continuous wave, back in 2007? I believe they came of Blackbeard's Conquest. I'm not sure if CRBenny has had the same issues, but I been totally unsuccessful finding a cure. Maybe one of these guys can chime in if they have an y thoughts.
My first few sea trials were in Fall of 2008, and I was really happy with the performance and numbers. I had not yet finished the wood, railings or electronics at this point. Some life things got in the way and about 2 years later I finished the boat. After that I've only had 2 short trips that did not result in a "limp home".
I always flush religiously and used to store the boat in an enclosed warehouse. Only Yamaha parts and lube with lots of care given.
I have tried everything mentioned in this thread, new poppets, thermostats, water pumps and fittings. The heads have been off twice and everything inside and out looks clean and clear. I have done work myself. Had work done by both Yamaha and non-Yamaha mechanics. The problem always reappears. I recently took the boat out in a "maybe I fixed it" cruise, it had been out of the water for over a year. It starts great, idles perfect, accelerates well and gives the dreaded beep after 5-20 minutes at cruise speed.
I'm sorry I don't have anything helpful to contribute other than some shared frustration. I hope someone chimes in with a good solution that we both can learn from. This must be a known problem we share.

I spend my time on the water on a our Montauk that used to belong to deepwater and our 20 Supercat. It is a shame to have our beautiful Outrage wasting away neglected. Repowering is not a realistic possibility right now, an a true fix would be a godsend.

kwik_wurk posted 07-30-2015 06:42 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
Pardon my ignorance on the Yamaha engines, but what about replacing the temp sensor itself (and maybe parts of the associated wiring harness). I have had a temp sensor go bad on a mercury, and it was the first thing I replaced. (This was based on the fact the engine was cool, and the coolant water flow was cool and strong.

It seems the key 'other' pieces have been checked off (thermostats, and water pump).

jimh posted 08-02-2015 02:45 PM ET (US)     Profile for jimh  Send Email to jimh     
The term "poppet" refers to the type of valve, but it is often interpreted in outboard engine applications by some manufacturer's lingo to refer to a cooling system pressure relief valve. Compare at


The term "pressure relief valve" is more appropriate and more descriptive for a valve in an engine cooling system that opens at a certain pressure.

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