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

[brodmann]

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.

 

[KJM]

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.

Ever try running it with the thermostats removed? Did the old impeller have fins missing that might be clogging a water passage way? Maybe run some descaling product thru it in case you have buildup in any passage ways? I would stop running at WOT until I figure out the problem or you might create bigger problems!

 

[brodmann]

I've considered running with thermostats out, but wasn't sure if that was a smart thing to do Constantly overheating and backing off throttle to cool down is obviously not too smart either. Yeah, I limped all the way home yesterday at about 3800 RPM's (25 MPH), it was a relaxing, fun ride home and motor volume was low enough for me to enjoy some tunes from my bluetooth speakers on the ride. But, the water was so smooth that it was difficult not to take advantage of that and really open it up. I know I have to keep RPM's below 4000 to avoid the overheat, but I do want to get it fixed and know that everything is okay. I can't test anything I do to correct the problem while in the driveway on muffs though. And at my neighborhood boat ramp, I have about a 20 minute "NO WAKE" zone in either direction.
Thanks for the help, and I'll let you know how it goes.

 

[KJM]

I was also wondering if there was some way to blow out the water passages with compressed air? I read a post where someone used compressed air on the water intake tube while the lower unit was off. Not sure if thats a good idea or not. Maybe someone will have a better idea.

 

[99yam40]

inspect the plugs to see if any cylinder is running different than the others.

as said if you did not find any parts of the impellor missing to plug up cooling passages, I would drop the lower unit and run some Rydlyme marine descaler thru the motor for a couple of hours to see if that helps.