OX66 225 low speed overheat

[kingfish]

We have a 225 HP 2001 OX66 with 390 hours on it, and have been suffering an irritating problem for over a year now that our marina (remote in Canada and new to Yamaha) has not yet been able to solve. At dead low idle (500-600 RPM), in gear and moving, the starboard overheat sensor causes the alarm horn to sound after 3 to 5 minutes running. It is always on the starboard side, even after trading the sensors from one side to the other. If left at dead idle in neutral, the problem does not occur, at least it hasn't for up to 1/2 hour. Additionally, the horn will stop sounding if the RPMs are increased (in gear) up to only 700 RPM, after only a minute or so at the higher engine speed. The problem does not occur at any other speed, in gear or out.

I replaced the impellor last summer thinking it was a cooling water problem, and the new part made no difference. I'm thinking the next thing I should have done was to trade the poppet valves (not sure if that is correct terminology) from side to side and see if the problem followed the poppet valve, and if not then to trade the thermostats side to side to see if the problem followed the 'stat.

The boat is 500 miles north of me now in Canada as I'm back home in Michigan, and the marina up there is going to be trying for about the fifth time this week to track the problem down, in my absence. Any help would be appreciated, both for my benefit and to pass along to the marina.

Thanks-

[Ken]

kingfish,
I am having a similiar problem now with my C115 but on the port side. My temp gauge is reading normal (130) but the thermo switch is closing at 180 (I tested the thermo switch).
There is only about five inches of motor between the temp gauge sensor and the thermo switch. But between them is the thermostat. I am going to remove it next and test it. I recommend you do the same.
I figure it has to be either a bad thermostat - could be clogged with salt/sand/mineral deposits, or the water jacket of the head clogged with the same.
You might try switching the thermostats, but if you are going to the trouble to remove them, might as well test them per the Yamaha shop manual.
Good luck,
Ken K

[kingfish]

Turns out ( I found through an in-depth phone call this morning) that the marina installed new thermostats on both sides this spring - it seems like an unlikely coincidence that one of those would have failed or been faulty, especially on the same side we were already having trouble.

So I have cancelled my order for the marina to pick the boat up from the slip and start troubleshooting, and I have ordered 2 new pressure relief valves (what I may have erroneously called "poppet" valves), along with 2 new compression springs and 2 new gaskets. R & R for these appears simple and straight forward, and I think the next family party going up there should be able to make the change and see if the problem is fixed.

I sure hate the digital "idiot lights" we have for gauges though, and would certainly prefer the old fashioned analog gauges that actually tell you what the temps are that we're dealing with, at this juncture...

John

[kingfish]

p.s.

Having now re-read the thread further down the page about overheating, I'm struck by the possibility that I don't know all I thought I knew, yet.

Am I correct that another possibility in this thing, besides bad sensor, bad thermostat, bad pressure relief valve and plugged water jacket (which I am currently discounting be3cause this motor has always lived in the cool clear waters of norhtern lake Huron), is a bad thermo switch? And that the thermo switch is a seperate and replaceable parrt that can fail and cause things like this to happen?

John

[Ken]

kingfish,
You said in your first post that the temp sensors (thermo switches) were swapped out from side to side and that the starboard side is the one giving the alarm. These two switches are hooked up in parallel, and either one can sound the overheat alarm - how do you know that it is the starboard side giving you the problem?
Ken K

[kingfish]

Ken-

Firstly, thanks for clarifying that the temp sensors and the thermo-switches were the same thing - I thought this thing was getting away from me!

Secondly, I concluded the problem was on the starboard side and was not a problem with the starboard sensor because when the continuous horn sounded and the thermostat icon idiot light on the dashboard was blinking I removed the cowling and disconnected the black wire bullet connector from the port side sensor and nothing happened (the horn continued blaring). So I reconnected the bullet connector there and disconnected the black wire bullet connector at the starboard side sensor and the horn stopped; reconnected it and the horn started in again.

I then switched the sensors from one side to the other and the next time the horn went off, I tried the same exercise. It was still the starboard side that when disconnected caused the horn to stop. That doesn't sound like it describes two sensors wired in parrallel, or disconnecting either one would have stopped the horn wouldn't it?

John

[Ken]

John,
Now we are getting somewhere - you have done a good job of isolating the thermo switches - I would have done the same thing. And, yes, either of the thermo switches when hooked up in parallel like they are can sound the alarm - in fact, you could put as many thermo switches as you wanted in parallel, and any one would sound the alarm. Of course, you don't know which one is closing until you go thru an isolation routine as you have done.
Ok, let's re-evaluate the situation. It would be unlikely that both switches are closing too soon, but I would go ahead an do a temp test on the starboard side switch anyway - just to eliminate that possibility. (If you don't already have a Yamaha shop manual, get one for your model.)
Next, I would remove the thermostat and do a temp test on it too. Look at any possible mineral buildup in and around where the thermostat resides(even though the motor is run in fresh water only).
Next, replace the PRV, also called the bypass valve. Now, I went to the online parts catalogue for your motor (2001 225 HP) at boats.net and I can see only one PRV installed in the exhaust cover. Does yours have two?
Now, after having done all of this, we still have not eliminated the possibility of a water jacket restriction either in the cylinder or head assy - but if it were much of one, then the alarm would be going off at higer rpms also. This condition has me puzzled. Hopefully the PRV replacement will fix the problem.
Keep me posted - this is how you learn this stuff - and try not to get too frustrated - I feel the same about my overheat problem, but am determined and confident that I will find the solution.
Good luck,
Ken K

[kingfish]

Thanks Ken-

I have an OEM service manual, but it's with the boat, in Canada.

I can't get it through my head if the two sensors are wired parallel and either one can sound no matter which bank the problem is on, how it can be that disconnecting the port side sensor won't stop the horn but disconnecting the starboard sensor will, no matter which sensor is on which side at any given time.

John

[Ken]

John,
The only answer would be that the starboard side is heating up that sensor to the point that it is closing (around 185 degrees). See, both thermo switch wire pairs go all the way back to the control electronic lead pair wires. The control electronic pair of wires is just looking for closure anywhere down the chain of parallel connected wires. If any of the switches close, then the alarm will sound. You can remove any of the switches, and the control pair of wires has no idea what has happened, since in a normal mode the circuit is open. Tell you what - I will draw you a diagram and try to e-mail it to you.
- Ken -

[kingfish]

Ken-

Thanks - I'd appreciate that sketch; I have a full wiring diagram, but it is in the service manual up north (maybe I should get a second manual to keep here!)

I think the picture is starting to seep through this thick scull; the port thermo switch would not stop the horn when I disconnected it because it had never closed - it was still open. So all I know for sure now is that both thermo switches work, at some temperature, and that the overheating is *probably* on the starboard side.

I won't be back up there until late July, at which time if there's still a problem I'll work through some of the troubleshooting you've discussed here. In the meantime, I'll send the new pressure relief valve(s) up with my brother in a week or two. I think I can illustrate to him how to change the PRV out but I'm not confident of the rest.

I'll keep you posted here.

john

[kingfish]

p.s.

I also believe there is only one pressure relief valve on my engine, now that I look at the diagram more closely. Well, they're cheap, and I'll now have an extra after I install this one.

John

[kingfish]

UPDATE:

The indications are that changing out the PRV (Pressure Relief Valve) finally, after two years of frogging around, fixed the overheat problem. The boat was tied up to the dock and let run at dead idle, in gear, for more than 1/2 hour, and no alarm!

My brother made the change while he was using the boat, 500 miles north of here where we keep it, so this is sort of a remote control fix. He has brought the old PRV back with him but I haven't seen it yet; he said the new PRV (the part number supersedes the OEM part number in the parts catalog) apparently looks a lot different from the OEM, and according to him, has a larger mating surface and looks like it would provide a better seal. He said also that when he took the old one out, he could feel small pieces of something like carbon (scale?) that might have been keeping it from closing, which would be diagnostic.

I don't know when or why Yamaha changed the design on this PRV, but it sounds to me like any motor with the old style has a high potential for problems similar to ours. If I knew then what I know now, the PRV would have been the first thing I would have changed out. An Owners bulletin from Yamaha would have saved us some shekels and a lot of grief...

[kingfish]

5 years later low speed overheat again

The same symptoms have re-appeared. The entire water pump assembly has been carefully inspected and replaced twice, the entire water pressure valve assembly has been inspected and replaced twice, and the water pressure valve has been blocked right off completely and STILL overheats at low rpm. The water jacked does not fill until rpms are at 1000 or higher. It is as if there is a leak in the cooling system AHEAD of the pressure control valve. Seriously need help, Yamaha dealer is unable to diagnose and we don't seem to be getting Yamaha's attention. Is there a cooling schematic for this motor that would help us diagnose what is going on?

[boats.net]

Do a compression test to see if have a blown head gasket. May have to remove heads and exhaust plate to look for block water passages or holes corroded through head and exhaust plate.

[kingfish]

Thanks - have communicated to the Marina in Canada, and am getting ready to head up there. FWIW, pressure and volume (i.e., cooling) have continued to be way good through all other ranges of RPM ; only problem is at idle speed in gear or *****ing.