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Author Topic:   Yamaha 70-HP Two-cycle Engine Overheating
Trainerjim posted 08-08-2015 08:10 PM ET (US)   Profile for Trainerjim   Send Email to Trainerjim  
I have just finished a four month refinishing project on my 1989 Boston Whaler Super Sport. The boat had not been in the water for over a year, so I decided to do some maintaince on the a 2002 Yamaha 70 TR outboard engine. After replacing the impeller and oil in the lower unit I put it in the water today, August 8, 2015. At first there was not a steady stream of water coming out the confidence indicator. I was able to clear the hole with a wire and then had a strong flow of water coming out. After about two minutes at 3/4-full throttle, an alarm sounded showing that the engine was over heating. Before turning the engine off, I was able to assess that there was still a good flow of water coming out. If there is good flow out the confidence stream, doesn't that show that water is flowing through the power head? What else could be causing the over heating? A friend suggested that the thermostat could be bad. Is that a possibility? Thanks for any help. My wife suggests a good boat mechanic. She may be right.


tedious posted 08-09-2015 09:35 AM ET (US)     Profile for tedious  Send Email to tedious     
Your friend is right. The next [component to service] is replacing the thermostat.
contender posted 08-11-2015 04:38 PM ET (US)     Profile for contender  Send Email to contender     
You have been using your engine in saltwater, and the thermostat is probably a good thing to check. The salt binds them closed when not using them. I just replaced two sets [at a cost of] $30 for the thermostat and $2.50 for the gasket. [Replacement of the two thermostats] took about 10 minutes for me. You need a #10 wrench and a gasket scraper-cleaner. Use marine grease on both sides of the gasket when replacing the cover and clean and use grease the bolts as well--easy job.
Soho posted 08-16-2015 11:28 PM ET (US)     Profile for Soho  Send Email to Soho     
I ran my Yamaha 90-HP outboard engine in saltwater continuously for nine years. I then had to take off the head to clean out crud from the water jacket. Once you get all the bolts out--without breaking any--the rest is intuitive. If the thermostat change does not work, [removal of the head and cleaning of crud from the water jacket] might be your next option. Plenty has been written about this procedure.
Trainerjim posted 08-24-2015 10:43 AM ET (US)     Profile for Trainerjim  Send Email to Trainerjim     
Thanks for all your replies. I decided to take off the head and water jacket in addition to replacing the thermostat. The thermostat was covered in salt and corrosion and the housing was full of the same. After taking it apart I was very glad I did. There were a lot of salt crystals and corrosion throughout. I have been diligent about running on ear muffs after each salt water use, but the 13 years of mainly saltwater use took its toll.

I did a lot of scrubbing and scraping. Two of the water jacket bolts broke off and I had to take it to the shop to have them pulled, one came out, but the other had to be drilled and helicoiled. It went back together this weekend and ran great with no overheating yesterday.

After a complete wood refinishing, 8-10 coats new varnish, new teleflex helm and cable, engine de-salting, this Whaler is ready for many more years of service.

jimh posted 08-24-2015 10:49 AM ET (US)     Profile for jimh  Send Email to jimh     
Thank you for the follow-up report on the ultimate remedy to the problem that is the topic of this thread. It is much appreciated to have a follow-up report that gives readers additional information on the actual steps and repairs that produced the remedy.
RocketMan posted 08-25-2015 06:33 AM ET (US)     Profile for RocketMan  Send Email to RocketMan     
Aside from the noted causes and effects I'll add that my 1985 Yamaha 90 had the same symptoms once, which were caused by a thermal sensor in the head that limited operation to 2000 rpm. The 70 may have this same sensor and so could be a third potential cause though obviously not in this case.

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