Yamaha 70BET0 overheating alarm

[Yammie70]

Hi everyone,

I have an old 70HP 2 stroke (1997), it runs great but on my last 3 trips I had the overheating alarm buzzing.
I'm trying to better understand the problem (and make sure there is actually an issue). This is what happened in my last 3 trips:

• 5-10min after launching the boat and running at low speed (8kn zone), the overheating alarm starts and the motor goes in safe mode.
• I turn off the motor, remove the cowling (the heads is warm but not super hot), wait for a few minutes, start again and the alarm is gone for the whole day (30miles trips)

• The impeller has been replaced a few months ago
• The telltale is not super strong but it has always been like that and seems works fine

If there is the motor overheats, I don't understand why it is happening only once in the first 5-10min. I just have to stop and start again the alarm is gone. Then I can run the motor all day and the problem doesn't come back until next time.
Now, I live in Australia and it's summer here and when this happened it was pretty warm: air temperature at 90F/30C and water around 75F/24C, I don't know if this could be related...

If you guys have any idea what's happening, please let me know.

Thanks!

[99yam40]

I would pull the stat and check that it operates in the spec range.
pot of water and a thermometer are needed to see what temps it opens and closes at.

might be sticking closed but the opens .
while you have it out check to see how much scale ,salt, or corrosion there is inside the opening where the stat sits.

also I am not sure it your 70 has a PRV that could be sticking open that would cause a low RPM overheat.
something to check on if it has one

[Yammie70]

Thanks a lot for your help and sorry for the late reply.

I've tried the boat again yesterday and no alarm this time.
I'll still remove the stat and clean it anyway. I don't think this model has PRV, however after a bit of research I found this thread which seems to describe the same problem I had.

Apparently it could be related to water tube dampers and seals. I'm not sure I have these but I'll check and add them if there are missing.
I will update if the problem persists or is solved after that.
Thanks

[TownsendsFJR1300]

If the over heating started after the impeller change, it's possible something is missing, etc (could have something also missing/fell apart, etc).


The water tube is part #10 and "gaskets" on each end.

https://www.boats.net/catalog/yamaha...casing-drive-2





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