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'98 Yamaha 115 runs hot if over 4,000 RPM's

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Oldsaltydog

Regular Contributor
Late spring, I replaced the water pump, removed cylinder heads and exhaust cover and cleaned out all sediment (there wasn't much gunk in there), replaced poppett valve and checked thermostats. They both opened and closed at the correct temperature. Today, any time I got close to WOT, it would sound the overheat alarm. After this happened several times, I found that if I kept it at or below 4,000 RPM's, it would not have any problems. Cranks right up and runs great. I'm confident that it's not a lean condition. It runs perfectly at WOT, which is about 5200 RPM's. When the alarm goes off, it does appear to be actually overheating, but the tell tale stream is quite cool. If I back it down to idle and let it run just a couple of minutes, the alarm goes away and it's fine for a good 10 minutes of WOT before it goes off again.
Any ideas would be greatly appreciated. It's a 1998 Yamaha S115TLRX.
Compression is between 128 and 135 on all 4 cylinders.
 
Remove cylinder head covers and clean all scale away completely. If it's bad then I would remove the heads too and clean out block around the cylinders
 
Sorry, didn't know whether or not the head covers had been off. The only other issue can be water pressure as a result of leakage somewhere between the pump and entry to powerhead although I have never come across this. You do seem to have all bases, hence the lack of response to your thread.
 
Sorry for coming in a little late, You said that it is definitely overheating when the alarm goes off...How did you verify this?? temp of cylinders???
Just trying to rule out a faulty temp sensor. of if you could locate one cylinder way hotter than the others then you would have a starting point to look further.

I have been burned more than once by "new does not mean good" might be worth changing the impeller again just to confirm.

other than that, as stated above possibly a leak or partial internal blockage not letting water flow correctly to all or parts of the water jacket..

wouldn't hurt to try flushing it with a descaler such as rydlyme or barnacle buster. Plenty of videos on youtube showing how.
 
I have to share what the problem turned out to be. I posted this probem on several different forum sites and one guy with the same boat as me threw out a question that had me scratching my head. He asked if I had a plug in the bottom of my live baitwell. I had no idea why he asked. When I said no, he said "that's your problem!". He had the same problem and kept replacing impellers that were always burned up and had constant running hot issues. He realized that once he reached a certain speed and with a certain trim to the motor, he would quickly overheat. On one occasion, he had the lid to the bait well open and he heard an odd noise. The through transom overflow for the baitwell was sucking air through the open hole in the bottom of the baitwell causing cavitation and the water pump was sucking in air instead of water!! He also noticed that at the same time, his RPM's were increasing, but MPH's were decreasing!! As the propeller was working to move water, it was creating a low pressure area, and began scking air through the open hole in the transom all the way from the open hole in the bottom of the baitwell!!
Sure enough, my impeller that was only a few months old was basically melted. I would never have considered that scenario. Guys, there's a lot of ways to screw up your boat! Last fall, I took the plug out of the bottom of the baitwell to let it drain and to wash it out when I got home from fishing. Apparently, I never put it back in, so when summer time rolled around, I started having this problem, because there was more long trips and higher speeds getting to barrier islands for days on the beach and pulling grandkid on tubes. Overheating issues. So glad it's not a motor issue.
 
Oldsaltydog said:
I have to share what the problem turned out to be. I posted this probem on several different forum sites and one guy with the same boat as me threw out a question that had me scratching my head. He asked if I had a plug in the bottom of my live baitwell. I had no idea why he asked. When I said no, he said "that's your problem!". He had the same problem and kept replacing impellers that were always burned up and had constant running hot issues. He realized that once he reached a certain speed and with a certain trim to the motor, he would quickly overheat. On one occasion, he had the lid to the bait well open and he heard an odd noise. The through transom overflow for the baitwell was sucking air through the open hole in the bottom of the baitwell causing cavitation and the water pump was sucking in air instead of water!! He also noticed that at the same time, his RPM's were increasing, but MPH's were decreasing!! As the propeller was working to move water, it was creating a low pressure area, and began scking air through the open hole in the transom all the way from the open hole in the bottom of the baitwell!!
Sure enough, my impeller that was only a few months old was basically melted. I would never have considered that scenario. Guys, there's a lot of ways to screw up your boat! Last fall, I took the plug out of the bottom of the baitwell to let it drain and to wash it out when I got home from fishing. Apparently, I never put it back in, so when summer time rolled around, I started having this problem, because there was more long trips and higher speeds getting to barrier islands for days on the beach and pulling grandkid on tubes. Overheating issues. So glad it's not a motor issue.
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