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Honda BF75D Overheating at full throttle only

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Ming15237

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New member here and am asking for advice on a friends boat. This boat has been in fresh water only. The engine as stated runs fine at all lower rpm's, but as soon as you give it full throttle it sets off the temp alarm and reduces rpm's. The thermostats have been replaced, the impeller replaced, and the oil is was changed. I have read about a a pressure relief valve being a possible issue, I don't know where this valve is located. I am an auto tech by trade so not afraid to do some digging in the motor to check it. I did get the DRHonda software to check the temps and as soon as temp sensors #'s 2 and 3 reach 175 F degrees the alarm sounds and engine reduces rpms and shuts off within 30 seconds. There is plenty of water coming out of all the pee holes, and there are no blockages on the intake grates. I also flushed the engine and no debris came out. Via the software the engine appears to have just under 1000 hours on it. Any Ideas?
 
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Looks like I solved this odd overheating issue. As I stated earlier I am a auto tech by trade, so getting a little dirty wasn’t a concern for me. So both thermostats were replaced, the plastic thermostat cover on the engine block was leaking water, so I replaced with a new cover which just happened to be made of metal. I also replaced the 3 coolant sensors as they were only $16 each to remove them from the possible mix. The pressure relief false is under the lower plastic cover on the port starboard side of the engine. I removed and replaced that as well. I next replaced the impeller and all associated parts (I bought it as a kit). The pee stream was good before, but it was WAY stronger when finished. The boat went out and ran for 3 hours and ZERO overheating issues!! All in all we had under $300 in parts, and labor was about 2 hours in total. Given I had never done this before I am confident I could now do it in an hour or less the second time around. I hope this helps some of you guys with high rpm ONLY overheating issues.
 
Ruigie said:
Thanks, I'm having the same problem on a BF75D 2007, what year's the engine you worked on ?
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2009 if I recollect correctly. Pay special attention to the thermostat covers, as they tend to leak water. The original covers are made of plastic, the new covers are made of aluminum.
 
Ming15237 said:
2009 if I recollect correctly. Pay special attention to the thermostat covers, as they tend to leak water. The original covers are made of plastic, the new covers are made of aluminum.
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Thanks for answering!
ok will check the covers and if in doubts change them!
 
So, if I were to do this over again, and in the interest of “less down time is a good thing”. I would order the following parts. Two thermostats stats (remember they are unique to their given location, I.E. different temps, DON’T mix them up!) two thermostat covers and seals. I would also replace the pressure relief valve and seal. It is the item in the first & second picture, and looks similar to a thermostat. As long as your impeller is good (two years old max imho) you should be good to go!
 
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