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Thermostat question - now with pictures

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Thermostat question - now with pictures

Old 05-23-2011, 03:01 PM
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Default Thermostat question - now with pictures

What would the symptoms of a bad or going bad thermostat be? I overheated while cruising over the weekend. Once it cooled down I idled for about 10 minutes to the dock with out sounding the alarm again. Trying to figure out if it could be the thermostat or what the proper course of action is.

Thanks,

Last edited by ndb8fxe; 05-25-2011 at 05:27 AM.
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Old 05-23-2011, 08:31 PM
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Stats would be the first thing I would look at. I have twin 2001 250 OX66. I have replaced all four stats with new ones once in six years. I take them out and soak in vinegar and they look like new. Always keep spares on boat just in case.
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Old 05-23-2011, 09:25 PM
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I'd suspect if it stayed cool at low speed but was overheating at higher rpm , stuck closed poppet valves would be somewhat more likely to be at fault, or clogged passages , but would still keep the t-stats & impeller on the list. If you didn't try powering up again, it could have been a temporary bag or similar obstructing the inlet too.
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Old 05-24-2011, 03:15 AM
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Originally Posted by TTaxi View Post
I'd suspect if it stayed cool at low speed but was overheating at higher rpm , stuck closed poppet valves would be somewhat more likely to be at fault, or clogged passages , but would still keep the t-stats & impeller on the list. If you didn't try powering up again, it could have been a temporary bag or similar obstructing the inlet too.
Its the other way around for poppets. It normally opens at speed, closed at low speed. So if it sticks open, overheating at idle is likely.
T-stats are a possibility. I would check them first as long as the pee stream seems to be strong.
If the pee stream is weak, pump is more likely.
Of course, internal corrosion, clogging, salt etc is also possible. Opening up the t-stat housings should tell you if that is the case. After that, most possible causes are not good news.
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Old 05-25-2011, 05:30 AM
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I got the left thermostat out. Here's a few pictures, there's a lot more corrosion on the housing that I would have thought. I flush this engine after every use. What do you all think? I can collapse the spring using finger pressure. I'll change them now that I am working on them, but do you think this is the problem?



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Old 05-25-2011, 05:52 AM
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Also, is there anything I can squirt down the passages to clean up some of the corrosion? I'm hoping to take the boat on trip to the keys this weekend.
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Old 05-25-2011, 06:16 AM
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Originally Posted by ndb8fxe View Post
What would the symptoms of a bad or going bad thermostat be? I overheated while cruising over the weekend. Once it cooled down I idled for about 10 minutes to the dock with out sounding the alarm again. Trying to figure out if it could be the thermostat or what the proper course of action is.

Thanks,
I would check/replace the pump impeller before I would look at the thermostats. What you are describing is a gone bad impeller.
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Old 05-25-2011, 09:09 AM
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Originally Posted by lostsinker View Post
Its the other way around for poppets. It normally opens at speed, closed at low speed. So if it sticks open, overheating at idle is likely.
T-stats are a possibility. I would check them first as long as the pee stream seems to be strong.
If the pee stream is weak, pump is more likely.
Of course, internal corrosion, clogging, salt etc is also possible. Opening up the t-stat housings should tell you if that is the case. After that, most possible causes are not good news.
Lostsinker- Yes , the spring-loaded poppets open due to increased water pressure at higher rpm flow to substantially add to the flow through the thermostat. If one or both were stuck closed as I mentioned, that could cause higher RPM overheat while the working thermostat is still able to keep things cool at low RPM when the poppets are normally closed anyway. It's just one possibility.
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Old 05-25-2011, 09:14 AM
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flushing the engine just gets the salt and other contaminants out...it doesnt add any anti corrosives to the system...a cooling system full of fresh water can corrode also...
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Old 05-25-2011, 09:41 AM
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Originally Posted by TTaxi View Post
the spring-loaded poppets open due to increased water pressure at higher rpm flow to substantially add to the flow through the thermostat.
Are you sure?

What motor (make and model) are you referring to?
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Old 05-25-2011, 12:59 PM
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Thanks guys, I replace the thermostats and ran the boat. I ran her pretty hard for about an hour with no problems. I'm not saying the t-stats were the problem, I'm leaning more toward I picked something up and it came off as I throttled back. Since I had the t-stats out I replaced them. Yamaha is very proud of their parts - ridiculously so.
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Old 05-25-2011, 01:26 PM
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Lots of misinformation on this thread.

The poppet valve OPENS during higher RPMS and lets the addional flow out of the motor. Since the impeller is spinning faster, there is more flow and pressure through the water jackets but you dont want TOO much flow so the poppet valve opens to relieve the pressure. So even if your impeller is not 100% you still may have enough flow to keep the engine at the optimum temperature. Which is why a idle overheat is likely to be a stuck poppet valve open and the cooling waters are not making it to the top of the heads where the temp alarm is.

The Tstats regulate the flow of water around the heads. So if the water starts to get too hot, the tstats will open to allow more flow.
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Old 05-25-2011, 05:10 PM
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Originally Posted by littletunny View Post
Lots of misinformation on this thread.

The poppet valve OPENS during higher RPMS and lets the addional flow out of the motor. Since the impeller is spinning faster, there is more flow and pressure through the water jackets but you dont want TOO much flow so the poppet valve opens to relieve the pressure. So even if your impeller is not 100% you still may have enough flow to keep the engine at the optimum temperature. Which is why a idle overheat is likely to be a stuck poppet valve open and the cooling waters are not making it to the top of the heads where the temp alarm is.

The Tstats regulate the flow of water around the heads. So if the water starts to get too hot, the tstats will open to allow more flow.
You got it!
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